If you pretty much want to keep what you have then pull the heads off dissasemble the short block and have .070 taken off the deck to get the pistons at zero deck. Use a .039 felpro head gasket. Doing this will give a little more compression and quench, both of witch will help hp and tq. Put in a nice roller cam about like the crate motors and a airgap intake. You would only have to buy a gasket set and about $100 machine work and 400 hp is very easy, it would probably make about 415. A little port touch up and a back cut of the valves should give the 425hp.

$800 is a little high around here but not much. $800 will get a late production low miles 360 mag motor here.