Unless you're planning to drive it hard, any GM automatic is probably going to be fine. If you start with a 70's Chevy pickup most likely it will have a TH350.
If you run Mustang II IFS you're going to have a tough time matching the front disks bolt pattern to a 70's Chevy pickup rear. Also, the pickup rear is going to be a little on the wide side. I'd suggest looking for a 70's Chevelle or Camaro/Firbird rear. That bolt pattern will match the 5x4.75 disks you can put on the Mustang II IFS.
