Would C4 swap for C6 Transmission?
No problem. The last factory production 429 Mustang was the 1971 Mach 1 Cobra Jet and Super Cobra Jet and the transmission choices was the Ford Top Loader 4-speed manual or C6 automatic. So a 429 or 460 transplant with a C6 transmission in your 1973 will be no problem. To do the transplant just look to parts listed for the 1971 429 Cobra Jet.