There have been 3 changes to the Model 70 since the pre-64 was discontinued.

Chg 1. It was redesigned into a push feed action because it was cheap to make not because it was subbed out to Japan.

CHg 2. they added a rail to the reciever to make the bolt travel smoother.
Chg 3. they brought back the pre -64 made some minor changes to the way it will handle a ruptured case, the claw is not spring steel so it will bend and remain bent and it is entirely made on CNC machines so it is probably better built than the original pre64.

There is no reason to build a custom rifle on the pre-64 action if you have a perfectly good post 64 CRF action..just replace the extractor with one of spring steel and go from there. Incidentally I have a new Model 70 CRF and MOdel 700 they can feed cartridges upside down too not that I have ever had the chance to do that hunting yet but who knows give me enough time anything is possible.