Yeah, the easiest thing to do would be put a newer (1980's are plentiful at the boneyards) 8 lug rear in.
You can't just change out the spindles and brake stuff. You'd have to change out the A arms and ball joints. To match a 5 lug, I think you'd be looking at disc brakes. And using a stock '64 rim would require 6 lug (or 8 for C20).
Save yourself a lot of time and hassle and pick up a newer 8 lug rear.
