How much power do engine mods yield?

Thanks for all the info guys! Torque is indeed what I am after...

The car will be used mostly as a commuter on 55mph roads with city driving making up 50% of the trip. Previously I used a 1961 Falcon as my commuter but that 144 inline was a dog... really got tired of people thinking I was an old fart poking along and cutting me off!

Interesting comment on keeping the auto tranny... would not have guessed that.