I am in thes ame boat, but I have a 72 with a cam'd L28, suspension mods...ect and a stock 73.
I would like to build one to sell and use the profit from that one to pay for the 2nd to be built for me.
So, I can tune up the 72 which needs paint and interior or put a L28(which I have a spare) or drop a SBC in the 73? Hard to say what would bring more money...