14 bolt FF vs SF

[hatzie]

The 9.5" 14bolt original equipment u-joint size was either 1350 or GM 3R
The 10.5" 14bolt original equipment u-joint size was either 1350 series or 1410 series

My guess is the 6 bolt 9.5 will have the GM 3R and the 8 bolt will have the 1350.
Likely the higher GVWR 2500 & 1 ton trucks had 1410 Universals. 8600GVWR and heavier rated?

You can get hybrid 1310 to 1410 or 1310-3R universal joints.

If you can get a look at the donor trucks SPID try to avoid the Eaton G80 Gov Loc axles. Not the end of the world as long as you aren't doing burnouts but they have a bad reputation.

If you're willing to move the spring perches and shock mounts you may want to measure the 10.5" GM/AAM 14bolt axles in the 2500HD 6.0L T800 trucks. They had very heavy duty rear disc brakes with good old fashioned two shoe & drum type parking brakes that used the inside of the disc hat as the parking brake drum.
The spring perch positions are not going to match a squarebody frame as is and I'm not positive you can move them out or in to mate em up. You'll need to measure one and do some head scratching. An OEM+ 4 wheel disc brake retrofit would be a nice nice touch compared to the other options out there. If the measurements will let you pull it off.