"91 LX Tranny problems
 

I just bought my first mustang last week. It is a 91 LX w/5.0. It is completely original with 43,818 miles on it. The only problem is with the transmission. It will not go into 3rd or overdrive (hope that is correct- I get 2 gears). I took it to a shop to have repaired, but I don't want this problem in the future. Next summer I am going to put more horses in it and I would like the tranny to handle it. Anyone have suggestions on how the current transmission can be made "bulletproof", or, should I look at a different one? Also, any suggestions on the route to take to around 350-400 horsepower?

its an auto?

you'd wanna build it... not sure on the exacts... don't know much bought autos but lots o folk round here do...

so build it
and
buy a higher stall torque converter

You can either go with the c4 if you don't care about having overdrive, or get a built AOD. Stock AOD can't handle too much in the line of power, it's not that the tranny blows up, it's just that the OD clutch burns out quickly if you do too many kickdowns from OD to 3rd with any respectable amount of power. Plus if anyone ever works on it, you have to make sure the line pressure that's adjustable at the throttle linkage (TV cable) is reset appropriately. Back when I had an AOD, there wasn't much in the way of building them up just yet, so I went with a built c4 but they now have some amazing AOD by Lentech or CKPerformance and TCI (I'm sure there are others out there). Of course, you could just order a T-5 5 speed swap and convert it over for the same amount of money