The 3R (S44) pinion yokes for Chevy 12-bolt are indeed very scarce. I couldn't find a single one, new or used, online. There are 3 common u-joint size pinion yokes readily available for the 12-bolt. 1310, 1330, and 1350. You apparently have the 1310 which is the smallest of the three. Big block Chevy's usually got the 1330 pinion yoke. The 1330 u-joint is very close in size and strength to the 3R (S44) that Pontiac used. If you need to replace your yoke while rebuilding the differential (old one grooved?), you'd might as well upgrade to a 1330 yoke as there's virtually no price difference. The hybrid u-joint you need for that is Spicer 5-793X. 1350 are very commonly used for racing, but would be overkill for street use.
Here's a 1330 yoke that uses U-bolts. There are also versions that use straps and bolts if that's your preference.
https://www.dennysdriveshaft.com/p37...y_12_bolt.html
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Hoping to finish a project while I'm still able to push the clutch in....
1963 Tempest Convertible (Ram Air IV w/4” stroke), TKX, 9-inch w/3.70 gears and triangulated 4-link)
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