i'm a 3.73 guy with a c4 all day long. imo if you start really modding the car and intend on racing it a little you will run out of gear far before the 1320. If i could talk you into keeping the aod talk to freddy brown trans in new york. they do aod's for randy seward's car in Tennessee. he drives his car to nmra and last year drove over 2000 miles to a true street race averaged 8's in the quarter and drove it home.
I figured you were possibly looking for a way to keep the car for yourself. If she will not mind, go ahead and get a shift kit and Trick Shift fluid and a hard line installed cooler in whatever auto you use. My gal likes to drive fast, but thinks my car is "tooooooo scary". She feels it will jump away from her around town. So it is all mine to play with.
As far as why people want Powerglides - I disagree strongly it is for less shifts! An auto can be programed (electronically or by hydraulics) to shift extremely quickly. The more time the car is kept in the powerband, the better. 4 gears will do that better than 2 until the 4 speed blows chunks. A CVT would be best, but as far as I know, no one has made one that will take a lot of power. The rubber bands snap.
I think a Powerglide (especially with a light, SFI case) is used for the same reason the 9" rear end is - mainly strength without a horrible amount of weight and LOTS of aftermarket parts availability. Unless you have enough torque that a first gear is useless, AND you have a slow shifting, stock tranny, (an unlikey combo), a shift is going to be quicker than any manual, and is not going to cost you time if it puts the car right back in the powerband. I read Porsche went tiptronic for that reason on their road racers. Now that there are 3 and 4 speed automatics that can handle some power, you need a radical combo of at least a very loose convertor, tires that get taller at speed, and a motor that spins to stupid high RPM to get a Powerglide to work it's best. Probably not a good combo for a car that sees the street. Some do it, but there are easier ways to go fast with a driver.
In my other classic car (it belongs on a different discussion site), I traded my GM 2 speed (even with a switch pitch convertor) for a TH350, and I love it around town. Mileage, acceleration and the fun to drive factor all went up. On the highway though, I still wish I had gone for the overdrive. I will likely have to go with a floor shifter to tell what gear I really am in and shorten the drive shaft, but it might be worth it with better brakes on that classic. It is more stealth than the 91 Stang, but not much slower. Unless it undergoes a number matching restoration (that is not likely, I want it to be like new, not like original) it will NOT have just a 2 speed Auto again.
^ I think there is an application for every type of trans all depends on what you're building. The powerglide was my best option because I would have a much harder time getting out of the hole with a 3 speed, the first gear ratio is too steep for my liking. I like the 1.80 first gear set much better with my current set up. No doubt a 3 speed car will go faster if it can 60 ft. If the 2 speed th/400 was in my budget i would have one but powerglide it is for now.
I do agree that no one would want a powerglide simply for less shifts and as far a car that doesn't have a lot of hp it would be so hard to go fast with a glide due to the first gear ratio. equate that to starting off in second gear with a c4 or similar.
The reason im pretty much stuck on the c4 is because its much stronger then the aod and can do everything i want it to stock. but ill still get the shift kit and valve body to make it that much better. i only plan on putting about 300 at the wheels. right now the plan is...
ported cobra upper n lower
ported gt40 heads(only because i already have them)
--With crane beehive or summit springs
small cam(undecided on wich one)
BBK unequal length shortys
4.10 rear gear....
im pretty sure that will get me there. plus the frame ties n all the suspension upgrades.
also today i traded a old set of long tubes for a chevy motor, for a brand new set of B springs. the headers were free so i think it was a good trade