Your making good progress.

The idea of doing a completely new rear frame isn't a bad idea, given the power your about to feed this thing. It would be a lot easier to set it up now, rather then to wait and take it apart to change it later. Might be a good time to upgrade the rear axle now as well.

I'm a Mopar guy, but there are not many heavy duty Mopar rear axle choices available these days. If you don't have a more suitable Mopar rear end then an 8 /4, a Ford 8,8 out of an Explorer has the 5 on 4 1/2" bolt pattern, can be had with disc brakes, and 3:73 posi rear end, can be bought for pretty cheap, and the width should be pretty close to what you need. You can buy a flange that will accept the smaller Mopar U-joint. You would need to cut off the coil spring brackets and weld on the spring perches for the leaf springs if you were going with leaf springs on your new rear frame, but there may be advantages setting up the new rear frame to take advantage of the 8,8 coil spring setup. The 8.8 will hold up a lot better to the 512 then an 8 1/4 will. Gene