dterry: cruising only! it will go kabang when you floor it. trust me, i know.
as for the yoke, all C6 slipyokes should be 31spline if I recall right, so maybe you can just pull one from the scrapper. The U-joint might fit, or might not.
you need to know:

1. Driveshaft side where the u-joint goes in:
- is is inside or outside snap rings (you can see the outside ones, the inside ones are c-clips just under the cap)
2. what is the cap diameter? 1", 1-1/16", 1-1/8" ?
3. what is the length overall of the u-joint axis in driveshaft? top of cap to top of cap.

The same measurements for the yoke
1. is it outside snapring? if yes there's a circluar groove just where the cap would end
2. what is the diameter of the hole where the cap goes in
3. what is the length of the whole u-joint (if outside snaprings from inside of groove to inside of groove)

once you have all those measurements, you will know what u-joint you need.

Kalli

 

Since the driveshaft is the right length, read Kalli's post, do some measuring and find the cross that fits the C6 Yoke on two pins and the 6 cyl driveshaft yoke on the other two pins. You should be able to find the dimensions on the internet somewhere, or if you have an old fashioned parts house somewhere in your area with a REAL parts man, he can find what you need given the correct dimensions. If that real parts man exists, take the yoke and the drive shaft to him and he will fix you up.

 

conversion u joints aren't available for the manual I6 driveshaft, but they are for the auto. for some reason the auto cars got bigger u joints than the manual cars. maybe the auto cars got the v8 driveshaft?

also, i wouldn't waste my money on them.

even if that 390 makes 200hp, it will do it by making an imperial crap ton of torque. torque is what kills drivetrain parts. the driveshaft will split or the rear end will grenade the first time you get any sort of traction. if you pop a driveshaft, there is a good chance of it buckling and punching through your floorpans. or flopping around and beating everything to death and taking out a tire with it and possibly the whole car.

 

I have a 390 in my '67. Do what others are saying, don't even run it to go get groceries right. Wait until you have all of the correct parts, then install everything together. You'll save a lot of time and money. Why half **** something just to redo it later. It ends up costing more money.

You can get pretty custom on Ford 9" rear ends from places like Currie [http://www.currieenterprises.com/] but they cost bucks. But you're in luck! The Ford 9" was used in a lot cars and trucks. You may even get the one that was in that T-Bird and swap it over with the engine and transmission...

 

most (all?) early t-birds don't have 9". it looks like a 9" but it's really a 9 3/8". nothing is available for them.

 

I promise you that there is a cross that will fit. You will just have to find the one with the correct measurements. They might not call it a "conversion" u-joint, but it is made.

That said, I tend to agree with all the warnings about connecting this rear end, although if you put the egg between your foot and the gas pedal and don't break the egg, you won't break the rear end.

 

The 65 390 makes 320hp and i know it makes a lot of torque. I was thinking about getting a 8.8 from a foxbody they are cheap at the junkyards around here and the parts are cheap too. Ive been looking for a 9" but ive found more 8.8s than 9"s. I just didn't have the money to get everything at once and i just wanted to drive it a little bit but i understand what everyone is saying. Im going to try and find a v8 driveshaft and a good rear end.

 

Another thing what all cars that have the 9" will interchange with my 68 mustang?

 

the only direct fit 9" is out of a '67-68. The '69-70 may work. The versailles, some granada, some monarchs are the correct width but require modification to fit.

the fox body (not sn95) 8.8 is nearly the same width as the '67-70 but requires quite a bit of modification to fit under the 67-70. if you go with one, i would consider at least welding up the tubes as well behind a big block. you'll also want to consider running an OD trans. most mustang 8.8s are 3.23.

 

I didnt know exactly what cars besides the 67-68 mustangs which would be a dirrect fit. Ive found a lot of 8.8s around here that are 3.73 but ive been leaning towards a 9" im just trying to find a good one and one that fits right.