Reply to "I'm on a learning curve, need a few answers."

Your comments regarding the exhaust leak lead me to believe the heads have the large 4V sized exhaust ports, so I would assume the heads are D0AE 4V heads with smaller volume (63cc) quench combustion chambers, machined for adjustable valve train (studs & guide plates). A head swap is very common. The heads should have a 4 cast into the corners, without a dot. Beginning 1971 a dot was added, the 1971 - 1974 4V heads had a "4 dot" in the corner.

The engine in your 1974 Pantera was originally equipped with D3ZE 4V heads with 78cc open combustion chambers. It should also have dished pistons. Are the pistons flat tops or dished? The 4 bolt block's casting number (near the starter motor) should be D2AE-CA. An engine with D0AE heads and dished pistons is what lead me to designing the 110 LSA Cobra Jet cam which I promote in my Q code engine thread.

I can't explain why somebody would install 2V headers with a huge port mis-match. Must have got a good price?

Once De Tomaso ran out of US manufactured 351C 4V engines, the factory began using Australian made 351C engines which did indeed have smaller "2V" sized exhaust ports. There was no "2V or 4V" nomenclature amongst the Australian made engines, they all had the smaller/lower port and smaller valve cylinder heads. The heads designed for the Australian 302C had smaller 58cc quench combustion chambers, whereas the 351C heads had about 75cc open combustion chambers.

The Boss 351 was manufactured in 1971, the block had a D1AE casting number, with 4 bolt mains. The pistons were forged with small pop-up domes. It had a very thick crankshaft damper, fully bonded to the hub. The heads were D1AE castings (66cc combustion chambers), machined for studs and guide plates. Dynamic compression was around 7.6:1. The valve spring seats were machined for cups, the valve springs were slightly stronger than other 4V valve springs, and the valves utilized single groove locks. A similar engine was manufactured in 1972, the differences were use of a 4 bolt D2AE-CA block, forged flat top pistons, and open chamber D1ZE 4V head castings (75cc combustion chambers).