If it's shifting too quickly and going into OD below 40mph, the tension on the TV Cable is too low (I made the same mistake when I first installed mine). Mine was because I had the bracket for the TV cable too close to the carb, so there was a little bit of slack in the cable. When I moved it back slightly and added a few millimeters of tension to the cable, then it wanted to run up to 2800-3000 rpm before shifting into the next gear - no matter how fast I was going. When I backed off the tension and set the cable to absolute zero/neutral (no tension, no slack), it's all perfect.
I read an article about how the factory set the tension on TV cables: they would put the cables into the throttle/TV cable bracket, adjust the slider so there was zero tension & slack, the clip the TV cable adjuster into place... no pressure gauge settings needed (since they didn't actually run the cars on the assembly line). This is how mine was done, since I had the factory cable along with the rest of the factory stuff, I'd left the adjuster alone and moved the bracket around. Since I set the cable into pretty much the same position as the factory would've done it, everything's now working correctly.
I would check that and give it a quick drive. If it doesn't help, then definitely take it back to the shop.
Also, I used the factory '89 Mustang shifter on mine, so the letters were all jacked up. I just rubbed 'em off the rest of the way and got some stick-on letters from Hobby Lobby. I drove it for a long time not even worrying about the letters, since the transmission's pretty standard - all the way forward is Park, next is Reverse (which can be slid into Neutral, which can be slid into Drive (w/Overdrive), then 2nd and 1st at the end (which can both be slid back into 2nd, the Drive from 1st).
The ignition lock-out on my steering column is to the left... I'd planned on zip-tying it up and out of the way (but never got around to it) - I never hooked mine back up because I forgot what that rod was for and kind of lost it after a few years of working on other stuff. Worse case scenario, if you have any issues of the key not working as a result of the lock-out ring rotating back into the key lock position, just grab the ring on the column and rotate it back to 'disengaged.' It won't affect anything else except the key lock, which will just make you scratch your head for a minute until you remember to rotate it back to 'disengage.' Unless someone as more information on its functionality, I don't see it as a safety issue or anything like that - more of a nuisance since you can't just remove the piece.
