I know somebody that bought a brand new 340 Demon with auto and 3.91's. All they did, and ALL they did was bolt slicks on it and it ran 14.20's factory bone stock with zero spin. But that isn't 13.9's. And that's not with 2.94's. lol
Speaking of tires, there is a lot to gain in terms of mileage with the right ones.
Since you are doing a regrind, maybe talk with Oregon about a dual pattern cam to help crutch the exhaust with your stock manifolds. Are the manifolds 318 or 360?
Whoops never mind about the regrind comment I see you have a comp cam already.
If you do nothing else to help compression at least run the Mr. Gasket .028 head gaskets.
Also another question- is your converter lock up or no? Obv 904 l/u would help mileage.