? dont know wat engine i should get?

i just bought a 1970 plymouth duster. it has a slant six in it right now but i wanna race it but i want a engine that i can get horsepower for cheap if possible i was lookin at the 340,360,440 but dont know wich one i want the 340 but thier hard to find for a good price

ok so lets say i just fix up the slant six how much do u think i can get out of it at the beggin i thought it wouldnt b worth it but now i am thinkin if i should just dump my 3000 in it like port n polish the engine n bye another manifold etc.
Author: envyedrico