['ol Pinion head]

The 3R 12 bolt pinion flange really isn't worth chasing, unless you have are involved in a very high point Concours restoration on factory 455 / 455 HO A-body car, and even then, most well versed national show judges wouldnt know the appearance of a factory 12bolt 3R pinion flange from an aftermarket cast steel Spicer 1330 piece. Would agree with tekuhn, the u-bolt style spicer 1330 12 bolt pinion flange is the way to go. Ignore the Parts Place listings & other sellers who are selling a spicer 1330 pinion flange with bolts & straps for the '70-72 455 application 12 bolt, that's incorrect.

Rebuild tips for those with a factory 12bolt with factory installed pinion flange.
During a build with a factory 3R 12 bolt pinion flange, one has to be very very careful reinstalling a factory flange, as if there is any "rock" of the pinion flange holding tool when bolted up (Kent Moore or aftermarket slotted style), one or two of the corners of the 3R 12 bolt flange can chip out, ruining the pinion flange flange. When rebuilding McKinnon 12 bolt rears which came with a 3R pinion flange, I go through the last phases of the build with a common 1310 12 bolt pinion flange. This means using the 1310 pinion flange to compress the new crush sleeve to "near" correct final dimension. Only after the differential has patterned out, do I use the 1310 flange to compress the crush sleeve. Then, I install the correct pinion seal (McKinnon 12 bolt & 3R pinion flange used a different pinion seal), & oh so carefully tighten the factory 3R 12 bolt pinion flange using a pinion flange holding tool of my own design, & the 3/4 sliding breaker bar & 3ft piece of thick wall pipe. Carefully installing this pinion flange in this way will result in no damage to the flange.