The pivot bolt does not go through the actual cases of the engine on an 86-87 TRX250R, rather it goes through and holds the rear engine mount. Two bolts then go through the engine mount and cases of the engine.

 

You should just have your steel swingarm shortened about 1.5". I had a bike with this mod and it had superior traction against anything at the track. you may want to consider the shock linkage mounts as well, I don't know if they are the same on both swingarms.