[dsraven]

not sure what your overall plan is but a lot of guys seem to want big fatties on the rear and so have tubbed their box sides and some have made the frame narrower. if that is your plan it would be pretty easy to do at this point of the build (the narrower frame). I wonder sometimes because guys will cut the front end off and weld in a sub frame, then cut the rear end off and make their own C notch or narrow the frame back there. really, the only stock part of the frame is where the cab is bolted down. overall, that is a pretty easy thing to match up if you just built a new frame, like you are doing.
I am sure you have a plan going forward but don't forget about the little things like where will the exhaust route through, where will the shocks mount up etc. I have seen a few builds where they get to the little things and go "ahh, crap. no space for the exhaust".
do you have a plan for the rear suspension? leafs, 4 link with coils or bags, etc? just curious. looks like a nice set up you have there.
another thing to consider is possibly.
good plan on the pin idea. I was thinking possibly a dowel set up welded into the frame on 4 corners so that a tight fitting pin could be slid into the dowel in order to eliminate the possible slop from a regular bolt or pin. some tabs on the outside of the frame would keep the jig effect square. looks like you have it under control though. please post pics as you go, you have me hooked now, lol. I was going to do that for my 57. I did a frame swap instead, with a bunch of other stuff added just to keep me busy for about 12 years now, haha. seems like something always comes up to take my time and shop space. i am sure you know the feeling.