But I'd like to point out that changes like port stuffers are a moot point on 351C engines that have other issues, such as low compression versions of the 351C, engines with too much camshaft, or engines with carburetors that aren't tuned well. Engine's like that have more important and basic changes that need to be made before things like port stuffers should be considered.
Port stuffers on the intake side:
The ramp in the bottom of the 4V intake port is responsible for the size of the port's inlet. It was placed there for a reason, to guide the air flow in the port towards the roof of the port, and thus make the port behave as a higher port (i.e. improved air flow). This same port design was employed in the 1965 427 FE engine's "medium riser" cylinder heads to give them similar air flow performance as the 1964 "high riser" cylinder heads. The SVO C302B cylinder heads also incorporated this ramp in the floor of the intake ports. By filling in the floor of the Cleveland intake port that aspect of the engineering that went into the design of the port is disabled, air flow shall most definitely be reduced. Filling the port also reduces the volume of the port, which has a detrimental effect upon one of the Cleveland's trademark properties, the ability to supply sufficient air/fuel mixture to a very large 2.19" intake valve.
In the days when racing parts had to be sourced from production cars, there was a limit to how high an intake port could be because the induction system had to fit under the hoods of production automobiles. So Ford's ramped intake port floor was a "hot" feature. But as the engine speeds of race cars increased, and as race teams competed for every small improvement possible, the ramped intake port floors were found to create a problem ... fuel separation.
When drag racers stuffed the intake ports in the late 1970s they had a very specific goal, to give the intake port a consistent shape and cross-sectional area, because the port's irregular shape was causing the fuel to go out of suspension at very high rpm (over 8000 rpm) due to centrifugal force. Increasing port velocity was not their intention for stuffing the intake port. Remember, these were engines that were launched at 8000 rpm and crossed the finish line at 10,000 rpm
. The port's "average" cross-sectional area was not decreased. After stuffing the port's floor, the intake port was re-worked to give it a very smooth consistent shape and consistent cross-sectional area. The port was raised as much as possible to regain air-flow that was lost by removal of the ramp. And port volume had to be regained in order to properly supply air/fuel to the 2.19" intake valve ... especially when that valve was being opened quickly by a solid tappet roller cam! Obviously the ramped intake port floor is a trick no longer used. Modern "racing" cylinder heads are not limited by the height requirements of production cars. The factories simply raise the ports as much as allowed by the rules of the sanctioning body (usually NASCAR). The intake port ramp was used in the intake ports of C302B heads to circumvent the rules, but those heads were outlawed for NASCAR use, Ford advertised them as being "too hot for NASCAR".
Now you understand why there's a ramp built into the intake port of 351C 4V heads, and now you understand why the ports were modified by professional race teams in the late 1970s. The question is ... will your 351C 4V street engine be improved by intake port stuffers? Although the iron 4V intake port entrance is large, the port is not that size throughout its length. The intake port's entrance has a cross-sectional area over 4 square inches, but the port's "average" cross-sectional area is about 2.9 square inches. The port has an irregular shape ... this is true ... but it is not too large. The size of the entrance is visually misleading. The port volume of an un-ported iron 4V head's intake port is only about 242cc even with the large entrance! If the large entrance required by the ramp were eliminated, the ports volume would be about 227cc. The port is tuned for peak horsepower at 6000 rpm, which is arguably right where it should be for a high performance street engine.
With the right cam and right intake manifold the 4V intake port performs very well as-is; it doesn't need to be "fixed". The things that are most often responsible for poor low rpm performance are (1) low compression, (2) too much overlap, or (3) a poorly calibrated carburetor.
On the other hand, if you prefer the power characteristic of a smaller sized intake port there's nothing wrong with that. We are all entitled to our preferences. Perhaps a set of 2V cylinder heads are in order?
Port stuffers on the exhaust side:
The exhaust port is a design compromised to fit within Ford's high shock tower style chassis. It has an unusual flat roof in the valve pocket area, followed by a bump in the roof. These features were intended to encourage exhaust gas flow to turn downward and flow into an exhaust header that turned abruptly downward thus hugging the side of the engine. The raised/rounded exhaust ports of the "port plate" modified iron cylinder heads, or those exhaust ports found on the SVO A3 cylinder head, where the preference of the engineers who designed the heads in the first place.
Stuffing the floor of the exhaust port should pose no problems, and may be beneficial, if the cross-sectional area at the outlet of the port is no smaller than 2.4 square inches; AND if the exhaust headers are designed in such a way as to compliment the reconfigured exhaust port and higher exhaust gas flow. If the headers turn abruptly downward and hug the sides of the engine, as some Mustang/Cougar headers do, I wouldn't use port stuffers. However Panteras may pose a better application for exhaust port stuffers as the headers will compliment the reconfigured port and raised gas flow better.
A word of caution: you can't "just install" exhaust port stuffers and expect an improvement. If a person decides to use them then the exhaust ports will need a bit of porting prior to installing them. The flat surface and the bump in the roof of the exhaust ports must be eliminated, otherwise the two features will counteract each other. The flat surface and bump would be encouraging exhaust gases to turn downward, while the stuffer is filling the lower portion of the port, and encouraging gases to take a higher path.
Do some testing before and after installation of the stuffers, and let the stop watch determine if the changes were worthwhile.