another tidbit i had forgotten about. [oldtimer's disease strikes again. ]
to the related topic at hand, did i miss just how much length needed to be added to the OP's driveshaft ?
maybe there is a factory shaft for a different application that might work for his needed length ?

I think it's only ~0.5" short
I expect to get my trans back next week. Once I get it put back together I'll have an exact measurement.

My friend was a master machinist welder.He made many drive shafts for cars and semis.Never thought about trying to lenghten one.Made one for my Jeep from oil well pipe.He would cut off the ends in lathe then weld in new tube.Made one for semi wrecker I knew of for sure.Guy snapped like 3 factory ones.His ran for 5 years and was still good when he sold the truck

Got my trans back, it will be awhile before I get it installed, but I think I'm ok.
The 2 main things I have going for me is this 518 yoke is 1.0 - 1.5" longer than most 727 yokes, and there's 1.25" of splined shaft beyond the case end.
The numbers in RED are if the yoke is pulled out an addition 0.5", looks like I still have 3.25" overlap on the splines.

Check my math, hopefully I'm looking at this correctly

When in doubt, replace it with a new beef up one with proper length one

Possible? Yes
Should you do it? No
Agree with Cab and others. Either get a new stronger shaft made or replace the tube in yours. If you break a driveshaft it will cost you way more than a new shaft.

I've never viewed it as achieving a minimum spline engagement. I've viewed it as supporting the u-joint as well as possible. That means getting it as close to the bushing as possible without bottoming out. I think 3/4" forward movement at rest is the general rule.

Shaft is basically new, got it 8 yrs ago, less than 10k miles on it.
It's a 3.25" DOM seamless with 7290 joints, cost me $400 back then

I don't mind spending money "if needed", but not looking to throw $500-$600 at something if not necessary.
Was hoping someone had experience.
If the trans shaft only has 3" of spline, is there benefit to have the yoke pushed in further.
I know it can't hurt, but can't see how it would help.

I would rephrase to say as far into the bushing as possible. Why? to limit the leverage by created by the unsupported yoke that may lead to premature wear of the bushing. Agree on the 3/4" or so

Help me out,
What and where is the bushing that will wear?
Isn't the fit and any play or wobble determined by the tolerance of the yoke to the trans output shaft?

The 727, 904, 833 and all the other Mopar trans that I have seen have a tail shaft bushing at the very rear of the tail shaft housing that the tranny yoke or front tranny ball and trunnion yoke slides into
it is there to help stop leaks and for aligning the yoke

No, as Cab stated it keeps the outside of the yoke inline with the splines on both

The bushing is at the rear of the extension housing, just in front of the seal. The bushing surface can be bronze or babbitt.

Okay. I guess I could have been more clear, but we agree. "...That means getting it (i.e. the u-joint) as close to the bushing as possible...."

I understood exactly what you said but wasn't sure the OP would so I was just make sure the OP did

Update, got the trans installed and I should be ok.
My measurement matches what the factory indicates, mine has a 1.5" pull back, in a perfect world I'd prefer only 1", but this will have to do.
Yoke has over 4" inside the tail shaft, bushing looks to be only 3" or 4" long, so that's completely covered

Just make sure that's not one of those Chinese tape measures 🙄, Also did you check the gravitational phase of the moon to earth before taking the measurement as it could be skewed.

OK, Hip waders off, was getting a bit deep there LOL

is that measurement picture with, or without, cheater glasses being used for focus ?