Head Milling.

I would do a couple of things if I were you.

1. Calculate the static compression ratio. That means take off one head and CC it with a burette etc. Using a dial indicator measure how far your pison is in the hole. Get specs on your pistons and head gasket. Do the math. You can find it in summits how to section.

2. Now that you know your correct compression ratio now you can make a decision. 10:1 is about the limit for pump gas. Aluminum heads you can go a little higher.

BTW you won't see much of a difference either way by simply varying the compression ratio. Try to get it in somewhere between 9.5 and 10.5 and you are good.

I agree, don't bother milling those heads. 10.3 ish is about what you can run on pump premium with a decent advance curve. That isn't bad.

A flat top piston with a quench chamber head will do that.

What's wrong with a nice set of iron 4v heads? There are quite a few cams that work well with them.

The 2v Aussie heads are over rated. If you look at them the intake ports go the wrong way. Anything over .500 lift is a waste with them. They simply stop working there.

The Australian 302C heads (2V heads with quench combustion chambers) have nominal 58cc combustion chambers, whereas the US 4V quench heads have 63cc (D0AE castings) or 66cc (D1AE castings) combustion chambers.

At 10.0:1 & 351 cubic inches, each cc alters the compression ratio by approximately 1/10th. In other words if the motor had 10.0:1 compression and 63cc combustion chambers, and you swapped the heads for the Australian 302C heads with 58cc combustion chambers (5cc less chamber volume), the new compression ratio would be 10.5:1. Approximately.

According to Ford milling 0.006" off a quench chamber head reduces the combustion chamber volumes by 1cc.

The actual compression ratio of your motor will be dependent upon the compression height of the pistons, the number of valve reliefs in the piston domes, how much the decks of the block have been milled if any, and the thickness of the head gaskets.

I am not aware of how you plan to use your car, but its relatively common to achieve up to 500 BHP from a set of ported Australian 302C heads. I know of 1 fellow who has a 600 BHP Cleveland using Australian 2V heads. But he's not telling people what he did to achieve that performance.

500 BHP in my world is a lot of horsepower, overkill in fact for a street car (i.e. using street tires) lacking electronic traction control.

If more horsepower is your goal, I would leave the compression ratio where it is (potentially 10.5:1 or more) and have the heads ported, then make improvements to the camshaft, intake manifold, carburetor and exhaust system.

As Doug pointed out, opening the intake valve of a 2V head more than 0.500" does not achieve more air flow, which makes the 2V heads good candidates for use with camshafts having longer duration and lower valve lift; i.e. flat tappet hydraulic cams.

I'd advise doing something to achieve better intake port air flow performance than what's available with a stock, unported 302C head. If you don't want to invest in having the 302C cylinder heads ported, then perhaps the 4V heads or a set of aftermarket heads may be more to your liking. The Trick Flow alloy 2V heads are decent performers, the intake port flows air like an unported iron 4V head. The Trick Flow heads are also available in a CNC ported version.

Again, we don't have any idea what you expect out of this motor, but I'd bet we could spec out something satisfying for you.

-G

500hp from Australian heads isn't out of the box. Iron 4v heads is right there for the taking. Is that too easy and you need something more difficult?

...358 Cleveland, 4V iron heads(Huge Ports, Huge Valves, Polished Valve Bowls), Hyd. flat tappet cam, dual quad Holleys, Roller Rockers, 10.6:1 CR...Equals 522HP 460 lb/ft Torque. The Biggest Improvement is when You Install a Roller Cam, The HP is Pushed Just Near 600 at 596 HP 497 lb/ft!...A Solid Flat Tappet Cam puts You At 557 HP 473 lb/ft...