The original compression ratio for that engine is right at 7.2:1. It's been calculated a zillion times here. The piston is down in the hole about 0.180".

Increasing the comp ratio from the original honest 7.2:1, to 9.5:1 increases power output by 8.7%. So a 350 hp engine turns into a 380 hp engine with no other changes.

The original 440s were dynoed and at the standard conditions of that test procedure they would make 335 hp. That is per an interview with an old Chrysler guy in Mopar Action years ago.

I agree with whoever said first thing is to buy a new set of Stealth heads. 440Source has one pair in their clearance section for $750 delivered. Those heads are worth up to 90 hp over stock cast iron heads.

Now that flow is taken care of, compression is next. The Six Pack pistons will get you to about 9.5:1 compression, maybe a little bit less. But they will also give you squish which will help by reducing negative work.

If you cant find a set of those then using the data from another post, look for original pistons for a 1967 or 1968 HP 440, they'll at least get you into the mid 8s.

With a heavy truck and low compression you need a short duration cam that closes the intake valve early. Either the .484 or .509 cam is way too big. XE262 Comp is as big as I'd go.

R.

PS: Thanks to Polyspheric (I think) for this useful page:
http://victorylibrary.com/mopar/otto-c.htm