Alright so I did a th400 in my 85. Debated wether get a new driveshaft one made
Or find one to get cut down. I got a driveshaft out of 90 caprice.
It's perfect as it fits my rear end and would just need shortening
And th400 yoke up front to work.

But after Many hours at the junk yard I found a long yoke.
I wasn't sure What it came from. It was just on the ground.
I kept looking at it and it look like it would fit a th400.
So.I found a BBC truck.with one. I put the yoke in and it worked!
Counted the splines and the matched.
This yoke was a longer slip one so I took it. Thought I could rig something up.

I got home got underneath the car and put the yoke I got onto
My driveshaft. Hooked it up to the differential and tranny .
The yoke goes in as much as the short one with the
Driveshaft on with no issues.
Lined up the transmission and torque arm and it all fit perfect!
Does this meen I can go this route? It's a lot cheaper then a new one and
Getting it cut down. If so What you joints should I use? I'm not sure
What the sizes are.

 

Sounds like you found a TH400 slip yoke and not a bolt on yoke. The bolt on yokes are most common in trucks with a hanger bearing between 2 driveshafts.

The proper length for the driveshaft depends on the yoke position. Slide the yoke into the transmission as far as it will go then pull it back out 1". That's where it should be.

Not sure what u-joint the TH400 yoke needs. It's probably a 1350 but could be a 1330. Depends a lot on what it came out of. The driveshaft on the car is probably a 3R u-joint but out of a Caprice, who knows.

Take the yoke to your local parts supplier and find out what u-joint fits it then find a conversion u-joint from that to 3R.

Easier to get a new driveshaft made to the specific length and getting 1350 weld ends put on it then using 1350 yokes on the tranny and diff.