Ummm - No... you really don't.
My 351C-2V is pushing about 400hp (at the crank), and is backed with an AOD and factory 9-inch w/3.00s. The AOD came out of an '89 5.0 'Vert (which came from the factory with 2.75s. I received everything 'AOD' from the car, except the output yoke and TV/Throttle mount (since the guy still needed those for his 5-speed conversion). I adapted the AOD shifter to fit my '71, made a TV cable bracket using a Holley throttle cable bracket, and adjusted the FMX cross-member to fit the AOD's wider valve body/pan. The only other mods I had to do were using a 27 oz adapter flex plate, installing a hybrid VSS w/19-tooth speedometer gear, make an adapter harness for the Reverse Light/Neutral Safety Switch, and track down a conversion U-Joint (to fit the AOD yoke and '71 Driveshaft). Everything else was straightforward with the '89 factory components: torque converter, TV cable, shifter, et al).
The internal gear ratios of the AOD are identical to the FMX, except for having the final Overdrive gear. Since the AODs typically ran in front of 2.75-3.00 rear gears, it's just fine with factory equipment - I seem to remember 5.0 Mustangs being pretty quick back in the day. Since my engine's putting out significantly more power than it did from the factory, it has zero issues stepping out and getting to speed very quickly. I can boil the tires without much effort at all, and it only turns around 1800 rpms at highway speeds (just like my other, more modern cars - probably even gets around 18-19 mpg as well).
Am I going to win races with my set-up? HA!! Probably not even close... then again, I'm not trying to prove anything. I did once get the jump on a '20 5.0 GT, and he said he struggled to catch up, if that says anything (my buddy, actually). For the most part, everybody that lines up next to me at a red light seems to just give nods of approval and nobody has jumped off the line yet (late model Corvettes and Camaros, mostly). I think the Mach 1 reputation, engine note, and fat tires out back probably buy some respect.