Greetings, from Upstate Western NY (Pittsford, a suburb of Rochester)! Out here we (also) have a 1973 Mach 1. Its powertrain is a bit milder than yours, where the prior owner replaced the factory 302 with a 1994 F-150 351W, using heads from a 1969 351W 4v. The compression with a set of forged aluminum pistons was kept to 9.5:1 due to the low octane ethanol free gasoline out here (89-90). He also swapped out the original 2.79:1 rear axle gear set with a 3.5:1 TractionLok unit. The original front drum brake system was replaced with a wrecked 73 Mustang's disc brakes and master cylinder. After the 351W was dropped in the engine produced 360HP. Not a ferocious amount, but plenty for some spirited driving.
We did some upgrades also, to include installing an aluminum radiator and dual electric fans. The original c4 began to slip with its 1-2 upshifts at WOT. Rather than rebuild the C4 we opted to swap in an AOD that was set up a firmer shoft than the factory setup. I also set the Throttle Valve pressure to 39 PSI vs the factory 30-35 PSI, in order to get firmer upshifts are higher road speeds on light acceleration. Our engine RPM at 60 MPH dropped from 2,850 to a mere 1,850, making the Mach 1 a lot more fun to drive for more than 20 minutes, as the engine running at 2,850 was fun for a while. But after about half an hour or so it grated on me. With the AOD I retain the lower end performance we had with the C4, then getting great performance in 4th/OD gear. With the engine built for higher than stock performance, the Mach 1's top end is higher as the torque and horsepower to overcome air resistance is on tap at higher RPMs and higher road speeds.
We also have a well preserved, true survivor, real barn find 1973 Mustang Convertible. It was literally barn stored out here for over 40 years. We have done a few modifications, not huge. The powertrain is the original 302/C4/2.79:1 configuration. The vehicle has an original 21,000 miles on it, and has its original exterior finish and interior panels. Although it is not m our most powerful pony car, it is still a blast to drive. As with our 73 Mach 1 we added a Classic Air air conditioning system, which makes both cars a lot more fun to drive in the warmer and hot months.
The rest of our little fleet of pony/muscle/sports cars is a 1969 Shelby GT500 (my favorite car of all), a 1997 Honda Del Sol SI, and a 2020 Shelby GT500. Sadly, Car Season out here is only from mid-April - early November. The other months bring snow and ice, and salt on the roads. So, the pony cars are in Winter Hibernation until later this year.
I look forward to seeing more photos and details of your car(s). Keep s