cutting down driveshaft?

[BerniniCaCO3]

I spent the last 3 weeks rebuilding a transmission, an AOD for my car. It came out of a 1980s mustang; guy on craigslist was swapping to a manual.
I went to install it tonight, and, guess what-- driveshaft doesn't fit.
I do not know WHAT chain of events lead to this (ending up in a mustang), but obviously, this is a lincoln or truck AOD. The tailshaft housing section seems to be about one inch longer, as is the tailshaft itself. I had no reason to have checked, as this came out of a mustang, so it shouldn't have been the longer transmission. But it is. Gotta move on and figure otu what to do next.

I can uninstall the new transmission, tear down both my new and old, and get the right tailshaft. That's a lot of work and wasted effort.

Seems the easiest is to have a shop cut down my driveshaft?

Can you foresee any issues with this?
Should I just have a shop shorten by exactly 1", and rebalance, maybe with new U joints for good measure (I'm pretty sure they're original, though they don't have any discernible problems-yet).

How much might this cost? If it's <$150, excellent, worth it compared to the alternative-- tearing down two transmissions!
And, just to make sure, there's no reason this shouldn't work?
And, if I ask them to trim my stock driveshaft by 1", that will work? Any other measurement you might go on?
For an AOD, is there any other length, other than 1" longer or not, to be wary of? My measuring technique was crude, consisting of a tape measure at an angle; though it does seem to be 1" too long, give or take a quarter of an inch.

[1980c10]

If your a good welder and have a band saw and cut Off wheel you can do it yourself. I have done it on a few trucks I've lowered worked fine for years. Make a mark on the rear yOke and the drive shaft so you can line them back up the way they came apart. Then take a cut off wheel and split the weld between the driveshaft and yoke, the yoke is setup kind of like a plug, you should be able to knock it out with a hammer after you split the weld. Then put the the driveshaft in a bandsaw and cut your inch off or however much you need. De bur the drive shaft, put the drive shaft in a vice and clamp it downlevel, line up your marks on the yoke and shaft lightly hammer the yoke in place, level the yoke to the driveshaft, then stitch weld the seam back up. Done lol

[Mercracer]

Leave the transmission in place. Get yourself a tailhousing and aluminum driveshaft from a 95-97 P71. Install both. Done.

[gadget73]

Lincoln Mark VI two door driveshaft will also solve this. Good luck finding one.

[BerniniCaCO3]

I will try calling junkyards tomorrow, but do not hold out hope for the right part, let alone at the right price. The yards here don't have many boxes at all.
Pickings are often meager at the pick&pulls and are not worth the gasoline to trudge out there especially if looking for something specific.

Found a highly recommended guy, independently recommended by 2 separate folk actually, who wants $55 to shorten, $110 to shorten and balance, and $200 to shorten, balance, and replace both u joints.
Probably fair. I may not get better, in this urban area; or a better price may come at the cost of someone inexperienced who won't do a clean job of it.
No, I'm not a good welder. I'd fuck it all up. I know when not to do something myself; and right now I NEED a driveshaft by Monday night to get my daily driver back on the road! Not in a position to screw around with finding rare junkyard parts, or experimenting with welding up my own.

But I'll try, and maybe get luck with a junkyard aluminum shaft.

However, if I don't luck out and need to cut down my stock shaft, I want to make CERTAIN. 1" shorter, right? Only two tailshaft lengths, and the longer is exactly 1" longer? I want to make sure I tell my driveshaft guy the right number; it would be crushing to pay $200 to shorten, replace u joints, and balance a rebuilt driveshaft, only to find out it should have been 1.25" shorter.
I'm not in a position to easily measure driveshaft length; guess I could take a jack out to where it's parked and crawl under, but it would be a sloppy measurement.

Finally, I'm wondering how this happened. Not that it will fix anything, of course. Guy I bought the aod core from pulled it from a 1984 mustang, that had had a 1991 mustang gt aod installed in it. The '84 originally had a manual (t5?), and he was switching it back.
Did the v8 mustang in 1991 have the longer tailshaft? Or was he misinformed (and it can't possibly have been from a 1991 mustang gt)?