What you may want to do is some fitting and trimming where you mate the later frame stub with the trimmed to fit frame end in front of the firewall pretty much as my buddy did a ton of Camaro stubs on TF frames.
Gray area being the TF frame.
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For that you have to measure your current static ride height of the complete 56 truck, measure the complete donor or a very similar trucks ride height and bottom off front crossmember to shop floor height and with the TF frame on stands at the desired final ride height and level you start fitting the stub up to it with the crossmember at the measured ride height. That way when you have them mated up you end up with a final ride height that is real close to exactly what you want without any other mods.
This is a 56 3100 frame that my buddy did for someone a few years ago. This one ended up being super low with the bottom of the frame the same height as the bottom of the Firebird stub.
That part isn't important except for what I said above that you figure out the final ride height you want and using the method I suggested mate the two together. Not this low I would hope but at the height you want.
Leaving enough of the bottom of the frame rail from the donor to have a line to put parallel with the bottom of the 56 frame will help have the stub mounted right.
