318 build suggestions?

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.
In my opinion, leave the stock factory brakes, unless the car is used for road racing or something. As for the rear end, it needs to be the easiest rear to deal with when it comes to replace parts in my opinion. That means no weirdness like having a different bolt circle than the front hubs or having to remember which ford exploder the brake setup it's from. That is how I'm mostly doing my car, so I can have a better chance of finding the correct service parts by keeping them in the similar year and in the family of the same ish models. These are just my opinions, people can set things up anyway they want.