Ford Repair: Transmission Trouble, metal shavings, tail shaft

Question
Hi, I have a 91 Mustang LX 5.0 that I put a used Tremec 3550 Transmission in with bell housing and appropriate yoke. There was a horrible vibration at 70-75mph thereafter. We found a bent rear mainshaft and replaced it with a brand new one. Now there is still a vibration. We checked/switched multiple driveshafts, peered into the rear-end, installed new motor mounts and had tires balanced with no avail. Do you have any suggestions as to what in the transmission it could be. We could see no metal shavings, or any excessive wear in the tranny except in the rear brass bushing which was replaced with the main shaft. Do you think it could be the tranny case, or some other internal component? The car that the transmission was in before was in a wreck, and that is how we think the main shaft was bent. We are at a loss, any suggestions would be greatly appreciated. Thanks so much! And you can post this, I don't see a box to check for it.  

Answer
Hello and thanks for the question...I'll try to help.
 Have you checked your drive line angle? Also have you switched out the cross-member and tranny mount? Have you checked to make sure that the transmission is in the center of the vehicle, not just where the cross-member puts it?
 You also, might want to try making a tail shaft plug for the tranny and run it with out the drive shaft, to identify IF the problem is in the transmission or not.
These are just a few ideas.
Hope it helps,
Erik