Cool car and one of my favorite engines, my thoughts are forget a frame swap, your frame fits on this one and fits one of the best motors ever made in my opinion, keep in mind the words HEMI do not have much in common with the word cheap, anyway sort or how but they are amazing motors. If suspension is a worry consider a front ifs kit that fits otherwise just rebuild the factory (add disc brakes). Chrysler introduced the Hemi in 1951 so not the first years just a early hemi.
My other thoughts are if you did not spend your whole 3k budget go snag a cheap cavalier or grand am or something to get you around for a grand and take your time on this dandy instead of tinkering with a daily driver with no backup ride. Your parts can be found easily by advertising on the Hot Heads swap meet web site. Yes I would convert to 12 volts, heck anything generally that old needs rewired usually, insulation on the old wiring is generally degraded before now and most problems are electrical gremlins anyway and if new you know the wiring and you know its right, buy a nice kit and its money well spent. Check out Speedway motors for RUNTZ to convert the gauges Check out hotheads for your plug wires and engine needs as well, I am not an all original fan and thats just my choice, to me hot rodded is the way to go always, and with the Firedome in it your already there! The only other thing I would do is not call that beauty a 4.5 liter, that possibly could confuse one with a newer metric hemi, kinda like confusing Harry Truman with our current administration (not that the newer hemi is all bad either) Congrats nice find...