TKO 600 transmission sag on 91 mustang GT

what do you mean sagging? If you have the right bellhousing and the crossmember hooked up it should be right.

did u relocate your cross member i had too? and i also needed to buy a energy trans mount?

I've never heard of anyone needing any kind of washer or spacer to assist in the installation of a Tremec. But with that said you must obviously be experiencing something that would make you think that. Is there any way you can send a picture. As far as the installation, are your motor mounts fine, does the shifter seem to low inside the car, is it corrected mounted to the crossmember, is the engine itself leaning back? Are your top 2 bolts to the bellhousing tight? The only important thing is that everything is tight and you have the correct angle corresponding with the rearend. There really is nothing special to installing the transmission as far as fit goes. Its simply having the right splines on the shaft and clutch, using a Tremec supported bellhousing, and pushing the crossmember back in some applications. Other than that, it mine as well be the same transmission. Its so similar I dont know why they changed the bolt pattern other than to sell more bellhousings which is my belief since the only difference between the 2 is like one bolt is in a different location and its only off by an inch. Its pretty obvious it was to sell bellhousings. I've never had an experience using a stock tranny mount with a new TKO so maybe it does need an energy mount. The only reason I used the mount was because it came with the tranny and you mine as well since they are so cheap. Its worth a try for $40 bucks. If your using the old mount best to change it anyways since you went to a stronger trans for a reason. If you are using an aftermarket mount then without pics I dont know what to tell you. I'm using a 600 with a 460 and was told I need a one inch spacer which I didnt opt for so I should be the one with sag issues but I dont. And so if I dont see a need for one, you shouldnt.