76 duster originally 318- 4 spd 8 and 1/4 rear with 7260 u joints.
installing smallblock 727 so shaft needs to be approx. 3" shorter.
want to order a driveshaft. Strange has reasonable price on balanced shaft with 1350 ends.
is my 8 1/4 pinion a 27 spline? i need to get a stronger than stock 7290 or 1350 pinion yoke to match the new shaft.
can i install a new pinion yoke and just re torque ?

Why not just run a conversion joint on the back

Or get a driveshaft from Mancini racing that actually fits as is.

http://www.manciniracing.com/driveshafts.html

Very happy with my DS from Mancini

i would actually like to upgrade to the larger joint if i can as the motor is a 416 and get rid of the straps and go with u bolts.

You can remove and replace the yoke on a 8 1/4, but you need to remember it's a crush sleeve design as well.

Which means you had better reset the preload properly.

If you are just going to cruise this car then why bother with a 1350 U-Joint on a 8.25" rear??? Especially with a 416, because the first thing that will break is the 8.25" and when that goes the sprague will roll over in the 727 and you will have parts coming through the floor, but what do I know??

I would get the car up on stands under the rear axle "loaded" & confirm that the bottom of the rear shackles are pointing rearward a bit (can space the front shackles rearward as needed), might as well get it dead on while you are dealing with that area, then butt a yoke into the trans & get a C to C & subtract 5/8-3/4 from that dimention so that you get the correct shaft. Just me I would not mess with the pinion, some solid Neapco or Precision ujoints if needed would likely suffice. confirm that the front yoke moves smoothly thru both planes of travel and the rear ujoint moves smoothly thru its single plane of travel before installing the shaft. OT, a 904 is less rotating mass & is lighter & can be built just as stout.

i intend to visit the track now and then not just cruise.
seems to me running stronger joints are better than smaller ones.
i'm not as concerned about the 8 1/4 as its handled this motor for years.
and the trans has a billet drum and bolt in sprag.

my ? is the pinion spline count wich will enable me to order a yoke without first removing the old 1 to count or finding out after removal there isnt a 7290 or 1350 available. i have seen 27 and 29 spline .

That's axle shaft spine count, not sure it matters to the pinion yoke.

27 spline in your case, the 29 came out decades later in trucks.

10 (coarse) or 29 (fine) splines. EDIT I'm thinking if you take off the nut/washer you can count the splines (not sure if that will disturb the preload or not), others will advise on that.

Not applicable to the 8 1/4.

Seems they are 27 spline only on the pinion shaft.

My mistake

Not a mistake, a learning experience

Agreed!