Advice on 383..

With 98 octane fuel available, you can run more compression. The engine should take a 10.75:1 compression ratio without a pinging problem, maybe even 11.0:1. If you go to an aluminum head, you can bump that by another .75 - 1.0.

Typically there's a little ridge on the valve that impedes air flow. Consider having them back cut 30º to get rid of it.

If the fuel is unleaded, consider installing hardened exhaust valve seats. This will help durability more than performance.

I thought you might be giving something away with the old TM6. Turns out, you are. Maybe 2-4 hp over a newer design single plane manifold. That's not enough to get me to change.