Dave you dont have to box that frame, it has a pretty good "X" to it and should be more than strong enough for even a Big Block. you can use the rear springs if you want,
they make the rear shackels and front bushings for them. I used stock springs on my humpback and my brothers 32 ply 4dr but we have since changed my brothers car over to mini truck springs, they are wider and you can use less springs to get the same or better ride

the drum brakes will also work fine in that car but if you really want to going with disc brakes is pretty easy but is costly in my opinion Ron....