Welcome.
If you are chasing performance/lightness for your project, I would recomend the Post 81 twin shock. It is lighter than the pre-81, with nikasil bores, lighter flywheel, lighter starter motor and a little extra power, amongst other things.
Not that the pre-81 is a bad bike - mine is a 1979! Its just that it starts 30lb lighter whilst having the smaller frame.......
Rev. Light