1973 340, want more power.

Don't need 450 hp to go low 12's in a Duster. You should get to low 12's and have money left over from the 3 grand, means you've already got the 340. Need more info such as to what you have now.
Gears?
manual or auto? converter?
exhaust (headers? manifolds?)
Is the 340 healthy right now (oil pressure, overall a good running motor??)