As you are from California - I'll help you understand what you're looking at
that is a thing called "rust" - it's a terrible affliction that everyone else has to deal with and has been known to poison marriages, cause children to start cussing, and bankrupt entire families. While it appears all innocent on the outside of that panel, if you see it there, you should know that you will start finding it throughout the vehicle and it will jump to other, otherwise pristine vehicles.
I'm surprised they didn't turn you back at the agricultural station on the Cali border. With that said, it is good that you've brought that plague into California - they need a little rain with all that sunshine.
joking aside, since your parts-rig is 3/4 ton - you may be better off 'simply' swapping the running gear from one to the other. 3/4 ton frames are heavier-duty - add to that the 1/2 frames are prone to cracking at the steering box and it's an even better upgrade