It’s not going to be all that easy to find another 50-70 rwhp....... and you have to decide how much of that nice driveability and low end grunt you’re willing to give up to get more up top.
For “mid-300’s” at the wheels...... you’re talking about a 400hp at the crank combo.
If that’s really the goal, then I agree with pulling a head to verify the compression and overall hp potential of the heads.
If you don’t want to dig that deep into it....... cam swap, retest..... see where you’re at.
Which m1 manifold are you using now?
If it’s the aluminum dual plane that looks about like a 68-70 340 intake, I’d step up to an RPM A/G.
