So I am swapping my '71 Duster over to the 5 by 4 1/2 bolt pattern. I got rid of the 7 1/4 rear and bolted in a 8 1/4 from an a body. The car has a slant 6 in it currently with a 904 trans. It appears the driveshaft is now to long?? Any of you guys do this swap without swapping drive shafts?

From what I've heard you have no choice but to swap the driveshaft. But since your driveshaft is now too long, you could always get it shortened and balanced.

That is what I think... I measured the rear ends and I believe the 8 1/4 is about 2 inches longer than the 7 1/4. The current drive shaft is 51 1/2 and I need one around 49 1/2.

You could locate a driveshaft from an 8&1/4/904 equipped A body (I'm assuming '67 up A bodies have the same wheelbase) or as said (best way) have yours shortened/balanced/good ujoints. With the rear axle on stands (loaded) butt the driveshaft all the way forward into the trans/measure how much too long it is/subtract 3/4"/give that dimention to the shop.

If you want a stock drive shaft get one froam aspen or volare there are around 49.5" u joint to u joint.

You need to shorten yours by 1.60".

It is nice when you can just get one from another car at the junkyard, but the self serve yards treat cars like cattle. Cars are picked up by careless forklift drivers and the shafts are most often bent. Even a 1/4" bend in a shaft will result in serious vibration and early U-joint failure. The best move really is modifying your original. The driveline shop I deal with charges around $100 to cut, balance and install 2 new U-joints. I consider it a part of any car build or axle swap nowadays.

I went from a 7-1/4 axle to a 8-3/4 and shortened the driveshaft myself with help of a pipecutter, anglegrinder and welder. No balancing done. I recall I had to shorten the shaft about 2+ inch.
This was a good few months ago, still driving the car every day without issues.

I will be cutting the driveshaft again shortly when I swap the 904 for a A518 transmission.

I can get my driveshaft shortened and balanced for $76 total. I am going this route. Just for clarification what is the length that I need if I am running an 8 1/4 rear end and a 904 trans? From center of the u joint to center of the u joint?

Rather than trusting some measurements you got online, you should consider measuring your car.
That way, any subtle differences resulting from your car's long history will be taken into account.

The typical way is to measure from center of the where the rear u-joint cap will be, to the center of where the front u-joint cap will be, with the front yoke slipped out of the transmission about 3/4".

Better yet, get the recommendation of the shop doing your driveshaft on how to do this measurement.
(But I bet it will be close to what I just described.)

Yes, it is center cap to center cap per the shop. Any one know what the stock dimensions are for this driveshaft? My current drive shaft is 51 1/2 inches long and if I remove 1.6 inches as stated above I believe I will have the correct length. Can anyone confirm or deny this?

Yes, it is center cap to center cap per the shop. Any one know what the stock dimensions are for this driveshaft? My current drive shaft is 51 1/2

Hate to beat on this one, but when I swapped a 8 1/4 for the 7 1/4 in my M-body the factory dimension for the 8 1/4 was off by almost a 1/2"!
This was because MY car was slightly different. And it was a costly mistake because I had to pay to have the driveshaft cut/balanced/etc twice. $100 x 2
And I felt equally stupid because it took all of 10 minutes to measure it and get it right- and because I had the original shortened too much, I had to find another shaft for them to cut- which was another $30.00.

So- Learn from My Mistake- measure the length you need and don't depend on dimensions you get from the internet or other 'sources'. Your car- like you- is unique.
When you measure, be sure to put all 4 wheels on jackstands with the suspension in tension- like it is on the ground- or the measurement will be off. Then it is as described above- measure center of u-joint eye to center of u-joint eye and pull the yoke out about 1/2-3/4". This allows for in/out movement when the suspension flexes.
I did it the super simple way-
Took a piece of string, tied it on the yokes, marked it, then took it out and measured between the marks, then added something like a 1/2" for each side (total 1") (sorry don't remember exactly- this was 5-6 years ago) for each side to meet the middle of the joint.

Good Luck,
Hope this help,
CC.

Perfect, thanks!

Just found this little gem... I am pretty confident on the 49.90 length after measuring it in the car last night!

https://board.moparts.org/ubbthreads/upload2/4563759-PartNoDriveshafts.pdf