I know the firewall changed quite a bit for diff years and AC or not.. I think you will be OK, but I'd probably get some good pics for comparison before cutting. Only structural change I know from 73 to 75 was the brace from the flange between firewall and cowl to the fenders. The back flange was widened for the U nut, but don't think they changed anything at the front except the bolt itself I believe is a larger washer. Probably wouldnt matter anyway.
