Poking around checking my driveline and found my tranny yoke has a wiggle in it that shouldn't be there,  Took it to my friendly tranny shop thinking it was the yoke bushing in the tailshaft . Not so. Bushing good.  Output shaft nice and tight. Slide yoke on output either part way or all the way and it's loose on the shaft. Good wiggle. Tranny guy says the yoke is worn. Tranny is a muncie M-20 and it's numbers say it came out of a 73 chevelle. My drivehaft ( I assume its the factory 78 vette driveshaft.) has the u-bolt style u joints where it joins the tranny yoke. Questions are :1)  should a vette driveshaft yoke fit ok on a M-20 output shaft, 2) all chevy yokes in this era the same  diameter ( teeth numbers are different) 3) anyone know of yokes wearing enough to wiggle.

I'll reply to your question with a question.... Is it leaking?
If not, then you really don't have a problem. The yoke will typically have some play in it. The issue is how much? If it's leaking, then you would have a problem that needs to be corrected, but if it ain't leaking, I would say it's not really anything to worry about.
If your trans guy said the yoke was worn, but there is nothing wrong with the bushing, he's not telling you the truth, for whatever reason. If the yoke, a hardened metal part is worn, you can bet yer booties the bushing, a soft metal, is also worn....in which case it should be leaking.
The problem with changing the yokes on these cars is, there ain't nothing else gonna work BUT a Corvette yoke. And they ain't cheap these days.

I would suggest, if it is not leaking back there, and you don't have a vibration, fuggedabouit.

Thanks for reply Joel.  Nice and dry at yoke, no leaks. Tranny guy pulled bushing out while I was watching,  I was wondering how they do it, slid it on the yoke and it was tight. Put a new bushing and rear seal in and wiggle was still there. He said the same thing you did. Harden steel won't wear before a bushing. He knows the tranny and driveshaft are not the same age. He charged me twenty bucks. He's a car guy, even though its GTO's and Trans Am's.  I've got around 2000 miles on crate motor and it's runs great. Love the tranny . I do have a slight vibration over 70MPH is why I was looking at driveline. I had changed all six u-joints, new tires and aligned after rebuild. I forgot to change tranny mount so I may do that. Rear bearings seem okay. It's not a bad vibration. I may of forgot how vibartions were not even noticed back before these limos we drive today came out. I believe some GM cars , like the chevell's had pressed in U-joints on the front yoke and not the open end  ones like the vette.  Thinking the diameter was a little different.  I am putting in a new rear spring. Right now it has a 2 1/4 inch, 7 leaf. Going to stick with that since it looks original. Tranny guy broke the four mount bolts loose, shot some lube on them for me and retighten them so I won't have to struggle. He told me to be sure I have wheels on the ground before I do the final tighening on the four bolts when installing the new spring. The old spring is flat and even sags a liitle. Thanks again.

Went to order leaf spring and saw where some jobbers state the 7  leaf spring is a little heavy for street use. All I have to go by is I'm  riding around on a worn out 7 that I'm sure has lost over 50% of its manufactured specs. There is not a rear sway bar on my chassis. Thinking I should go with a 9 leaf, 2 1/4 if they make such a thing. Any thought?

I think most of the aftermarket springs will be far stiffer than original, according to the specs I've seen listed. Eaton  is the ONLY company that makes exact rated springs, but I have no idea how you could get the springs directly from them.
That being said, I would have to say if I was going to replace mine, I would find the lowest rated spring available, since I don't care about autocrossing, or any other kind of racing...I just wanna ride, and ride comfortably.

Thanks for the tip. Don't want anything stiffer so I'll probably do some research and look for a around 260-280. Thanks