TCP use to make them for Falcon and ranchero but ran into fitment issues. I think they were selling the same ones they used for the Mustangs. These can be used as can any sub frames for the early Mustang but the front of these are spread 1/8 to ¼ wider than those used for the Falcons. They can be easily altered to fit or used as is and just pacing a piece of steel plate along side the sub fame when it is welded in. http://www.mustangdepot.com/OnLineCatalog/Suspension/subframe_connectors.htm
Most guys seem to go with Global West. I just got mine from Crites Restoration. I test fitted them yesterday and I like the square tube, it matches the frame. Just make sure you go with weld in connectors. Bolt in units just won't hold up.
Here is my 2 cents. On my 64 I cut the ends out of the front subframe extensions. Predrill 3-4 1/2" holes inside and out of the subframe. I used 2 pieces 1x2"x .100" wall stiched togather side by side. Slide them as far into the front subframe as possible on there edge (4 vertical supports). Fully weld including plug weld the 1/2" holes you predrilled. You will have to cut the floor but is by far stronger. I can't help you on the back end at the torque boxes, as I installed a crossmember like a Thuderbolt.