Good advice.. I went the measurement method and rechecked my self SEVERAL times before doing the final welds. I used the factory support braces instead of an export brace. I would get the tower in place, tac weld after measuring, then checked all points of "contact" a few more times before moving on. Luckily, my car turned out fine and I was able to get a very good alignment out of it. The car actually drives the best out of all my Mustangs. :-)
I welded the towers first , then the motor mount sections, the rear aprons, the front radiator support, and finally the front aprons. Hope this helps..