[i heart square bodys]

i have an 86 Chevy reg cab long box with a th350 tranny in it, im swapping it out for a 91 th700r4. the issue i am having is that the 700r4 has a short tail shaft and the 350 has a long. it is a lengthy process to have them switched out so my thinking was to get a drive shaft from a reg cab sort box truck that's post 88 (short tail tranny) . is my thinking correct ? one other issue is that the 700r4 is an electric speedometer. can i just call my local tranny shop to get all the pieces to switch is to a mechanical ? what do i all need to do so?

thanks !!