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Any reason I shouldn't have the original shaft shortened vs going new? New engine has over 400 hp and 400 tq where it will be living in the rpm range

Is it a 1 piece shaft? I would bring it to a driveshaft shop and just ask them.they are usually very helpful and wont just sell you on something you dont need. Some shafts have joints that are bigger than necessary stock and some are outdated and tiny.

Yes. Single piece.

Generally speaking the overall diameter is determined by the length. Anything over about 57 inches will be a larger diameter. As barn find says a good shop won't steer you wrong.

I had my '66 one piece shortened. Installed a new front yoke & u-joints. I'm comfortable with it, but ,of course, time will tell. It's not like we'll be drag racing these trucks.
My mild 454 probably has about the same torque as your 383 but less HP.

Pulled my measurements. 53 5/16 center of yoke to yoke.

Looks like the joints will be different front to rear. Shop said to measure with yokes at their bottom and they will subtract as needed.

Front/rear u-joints being different is pretty typical. I ended up getting a conversion joint on my last custom job. One side of the cross was completely different from the other side.

I did that to get a larger tube and use the GMT800 front yoke on it while not having to change the rear axles.

Ferris Bueller I,ve made about a pickup load of driveshafts in 60 years,mine,friends,customers,always used shafts mostly shortened not many on high horsepower applications. Didn't balance much,if it's straight it is balanced,gotta be in phase. Learned to use an Allemite strobe light wheel balancer for fine tuning on high rpm drive shafts.

Driveshaft ended up being bigger than I remembered when I dug it out. I have no question about it handling the HP and TQ.

Take your driveshaft to the shop and see what they say. They may shorten it? But..... I took a drive shaft to Driveline Service here in Pgh Pa not to long ago, to have it shortened, but wasn't going to be cheaper in the long run. For $200 bucs, they made a new one to my length,with different u-joints, and attached my yoke to the front 1. Balanced, painted black, and picked up the same day. What more could you ask for? The application was a T5 behind a 8BA to a 47 banjo rear end. But that shouldn't matter. Find the shop, go in person to talk to someone like people are supposed to, and you might actually get what exactly you want. Not to mention, a better deal than you think. Good luck 👍

glenn's towing Our local All-Tran is like that ,whipped up a driveshaft we had it in truck pulling the next day !! Same guy since we were all young !!

glenns towing We went straight to the horses mouth at our local (old) all-tran,he made us a shaft fit and finish was great !! Sorry forgot I commented to you earlier.

Great. Glad it worked out.

Glad you got it all fixed up. One thing I noticed in previous posts that I honestly didn’t understand until a driveshaft guy explained it was this…… we all think of the driveshaft capacity as X amount of torque but in reality a 50hp truck can break a driveshaft crusin down the street…..its all about driveshaft RPM and understanding how a certain shaft will react to it. Think jump rope because thats what the shaft is trying to do under the truck.

It is a bit more complicated than that. If the shaft is perfectly straight, you can spin it to any speed without it whipping and breaking. That said, perfectly straight is a theoretical construct and impossible in real life.

So when you have a driveshaft that is damaged, an engine of most any size can break it.

Whipping (what you describe as a jumprope) is caused by centrifugal forces acting on an imbalance in the shaft.

The issue with high torque engines is that they can apply enough torque to twist the driveshaft tube itself, which results in small bends, making it out of round, leading to whipping from the centrifugal forces.

As it whips, it gets easier to bend, so it gets further out of round and the problem makes itself worse until the shaft fails.

Yea well that’s basically what i said hahaha. The perfectly round driveshaft is also basically impossible. You would think it is but if you slap a dial indicator on one under your truck you would be surprised how much it moves. darn DOM tubing.