agreed, if the 'tuned' 6 runs well i'd spend money on the rear end, front brakes and suspension before thinking about the motor swap. re the rear end have a look at ford exploder axles. 31 spline shafts, lsd and disc brakes are stock. cut the casing long side to swap in a second short shaft (cheap as chips from rock auto) and it's almost exactly the same width as an 'a' body 8 3/4. it's a common swap that's well documented on here.
once the car's built for a more powerful motor then save to build one right, once.