Okay here we go....
You're going to have to do a bit of investigative research here... There are MANY MANY MANY U-joints... Nobody can answer what you have if you don't know certain measurements. Nobody can guess this info.
You're going to have to have some MEASUREMENTS for what you have...
Like:
How many splines are on your 700r4 tail shaft...
This trans takes a slip yoke... The yoke itself needs to match up with whatever end you choose for your driveline. In other words - you might choose a 1350 U joint - with a 30 spline slip joint (this count needs to match the tail shaft). This slip joint can be bought in various lengths.
NEXT:
You're going to have to identify the pinion yoke on your rear end. There are various sizes here - easily identifiable by SIZE. Size being the diameter of the cap - and the width of the saddle... it could be a 1310 or a 1350 or a ??
NEXT:
With the car resting on jack stands placed under the rear axle... you only want 3/4" of an inch of the slip joint "exposed" (that would be the machined surface). Too long and it can kill your tail shaft bearings and give you a nasty vibration etc. The tail shaft inside the trans - already has a length that is "unsupported".
NOW -- there are MANY drive line sources on the internet. Start with DENNY'S DRIVESHAFTS
http://www.dennysdriveshaft.com/index.html --they have TONS of info about u-joints - and the way to measure your drive shaft length etc. There's also a FAQ (Frequently Asked Questions) section detailing the difference in yoke sizes as mentioned eartlier. I've just given you a little FOOD FOR THOUGHT with my info.