i have a 65 mustang 6cyl auto i changed the rear diff and but one from a 65 6 cyl std but the drive shaft rear joint is not the same it`s smaller then the auto can i change just the u joint or do i have to get the drive shaft end replaced

 

you can change just the u joint. take careful measurements of the cups and the width of the new rear and match that required for your driveshaft. Locat stores should have that for $20 or so.

 

right now the drive shaft joint is small on the drive shaft part and bigger for the yoke side but i need small for both end with inside locks

 

Remove the pinion flange from your old diff. Get a new seal and pinion flange nut. Remove the pinion flange and seal from your "new" diff. Clean the threads on the pinion. Install the new seal, pinion flange, and pinion nut, and torque to 125 ft-lbs. You can do this whole job in under two hours.

BTW, the typical C4 200 diff was the 2.80, while the 2.77 usually had a 3.20, if this is true in your case you should notice sportier acceleration.

 

ya either just change the pinion as 2+2GT says, or measure the driveshaft side and pinion side.
since you say both are inside clipped with a caliper:
driveshaft:
- measure the diameter of the hole where the u-joint sticks in
- measure the length from inside clip to inside clip
pinion;
- measure the diameter of the half hole to see what fits in there
- measure the length from tab to tab for the length of that u-joint axis

 

Thats interesting that your having an issue with that. Back when my car was still an I6, we had to swap the rear end out for one from an automatic car. My car was a 3 speed standard then. We didn't have any issues like that.

The comment about acceleration is correct. When I went from the 3.20 gear to the 2.80 gears, it completely slowed my car down and made it a dog. It was hard to get going from a stop in 1st.

 

flange from one diff does not fit on other diff and both diffs are from a 65 tries to find a joint to fit nothing so far so i put the old one back in for now need the car for the atlantic nationals

 

I have made this swap several times. I cannot imagine why you are having trouble swapping them. Did you get the correct seal?