The only parts that I know are interchangeable would be the knobs, pots and output jack. The top nut might be, but I'm not sure.
The cases are the same. The T-40 case will fit the T-20 and T-45. I'm guessing it will fit a lot more early Peavey basses as well.
I actually got my hands on a T-20 for the first time today and was going to buy it but the neck is too wide for my tastes. The T-40 is too wide as well. I'm better suited with something more narrow like the Fury or Dyna.