For what it is worth, I installed 2 sets of frame connectors in Mustangs, one was a 66 fast back with a 460 and one a 67 notch back with a 428. The only good way is to notch the floor and weld them in solid. Any thing else tends to defeat the purpose. On the BBF 67 I tried the straight bolt in ones first which used a 2x2 x1/8" wall tube, but it still allowed the body to twist badly. We even tried to fish plate and bolt through the floor to it but it actually ripped a couple of the plates loose. We finally made a new set of 3/16 wall 2x3 tubes, scribed along both sides of the 2x2 rails, notched the floor to match, and welded them in solid. You loose some of the rear foot well but you won't have anyone in the back seat anyway. It made a huge difference in the launch characteristics of both cars.
On the 66 we had to fab new torque boxes anyway so we just made new front rails and extended them all the way back notching the floor and incorporated new spring cups.