So you want a torque monster motor, that produces hp down low.Now when you say you have got your hands on a 4 bolt main motor, do you mean just the bare block or a complete engine ? Does the block need a rebore ? size of the block bore now ?

How much mechanical knowledge have you got ? Not knocking you, just there are a number of options some are harder to do than others if you plan to do it all yourself. Because everything from the carburetor to the pistons, head type and cam MUST work together.