7.25 swap to 8.25, Another Driveshaft Question

[PSUKev]

I've already searched and either I'm not entering the right search info or I'm mildly challenged but either way I haven't found the answer to my question yet, so here goes...

I'm rebuilding my 69 Barracuda fastback and I've chosen to rebuild my 318, beef up my A904 and replace my rear end. I'm upgrading from my open 7-1/4 to a 3.55 sure grip 8-1/4 out of a '73 Dart Sport 340 that I picked up from someone here on the board, no other changes are planned. I've opened up the holes on my spring plates to receive the wider, 3" U-bolts, had my old springs re-arched, flipped the front spring mounting plates to get a little lower and my only question is that if all I changed is the rear end and I use the illustration below I should only have to shorten my driveshaft the difference of 11.69" - 10.09" = 1.6"

Is this logic and 1.6" calculation correct or should I wait until the car is totally reassembled and actually measure what I need rather than calculate it? Thanks for any help.

 

[Moparmarcus]

I would wait and measure it, that's the only truly reliable method. I went through the same thing when I change my Scamp over from a /6 with a 7 1/4 to a 360 with an 8 1/4. The length of the driveshaft ended up being 49 3/4" from u-joint centerline to the back of the output shaft. At least that's the dimension my driveline guy asked for. It fits well and it only took him 2 days to cut, reassemble and balance the shaft.

 

[PSUKev]

I will probably wait and measure it like you recommend. I'm trying to do as many items concurrently to cut down on the time to finish my car and I thought this might be something I can have sent out and done while I'm doing the bodywork so when I'm ready to reassemble I could just drop it in place but I agree the only way to truly know it's right is to wait until the car is done and at ride height and fully dressed weight.

Answer me this though, is my logic at least correct? If I hold everything else constant (engine/trans) and I'm only changing the rear end my 1.6" calculation should be correct, right?

 

[Supershafts]

Wait and measure