And for posterity (and those crazy enough to try the same thing I'm doing), here's the info on both the 66 1/2 ton and 52 3/4 ton U Joint since I know good and well I got the 52 info from someone on here:
66 side: Universal Joint: Rear Drive Shaft; At Rear Axle; Rear Wheel Drive; With 3.219 Inch Outside Snap Ring Dimension; 1 Piece Drive Shaft; With 1.062 Inch Bearing Cap Diameter; Greaseable; With Outside Snap Rings; Diagram Position: 9
https://www.oreillyauto.com/detail/...3925/1966/chevrolet/c10-pickup-rwd?pos=2Precision 331 for the 52:
Brand: Precision Universal Joint
Manufacturer's Part Number: 331
Part Type: U-Joints
Summit Racing Part Number: PCN-331
Universal Joint Style: Spicer 1350
Greasable: Yes
Solid: No
Bearing Cap Type A: Round Plain
Bearing Cap Type B: Round Plain
Bearing Cap Diameter (in.) Axis 1: 1.188
Bearing Cap Diameter (in.) Axis 2: 1.188
Universal Joint Material: Steel
Quantity: Sold individually.
Notes: Premium universal joint.
https://www.summitracing.com/int/parts/pcn-331/overviewCombo of the two ends up being a Spicer 5-460X.
