How much power do engine mods yield?

There is an engine build matrix on slantsix.org. Pick one and follow it. They are proven recipes.

A quick and dirty recipe would be a stock 225 with a Supersix manifold, Autolite 2100 1.08 carb with #48 jets, recurved distributor with initial at 15* btdc, 30* total, all in before 3000rpm. A 2 1/2" exhaust from the stock manifold to the bumper. A TransGo shift kit in the transmission.

The 2 biggest improvements were the exhaust and the shift kit. This combo drop 2 seconds off a '73 Duster.