I'm really trying to cheap out on my build and was curious if i could disassemble an 89-91 RX7 halfshaft and use the splined end off my 1991 Halfshaft? Would this end up being the same length of the 95+ shafts?

 

I used the 1.6L hub-ends on my 1.8L halfshafts,

but yes it should work since:

"the 86-91 NA RX-7 halfshaft is similar to the 94+ Miata halfshaft with the same ~25.25" length and the same 94/95 Miata stub shaft bolt pattern of 4x110 but it uses a larger hub shaft diameter of 28 spline by 29mm"