I would not mess with the brackets on the bottom of the car. Just modify the double hump crossmember as follows:
1. Remove the 4 rubber bushings from both ends of the outer mounting tubes.
2. On each side there is an 'inner' steel tube that mounts inside and is supported by each bushing. Remove the inner tubes (2) from the bushings. (They may be rusty and you may want to clean them up a bit with a whire wheel, etc)
3. Using a fine tooth hacksaw cut the outer mounting tubes off flush with the sides of the crossmember stamping.
4. Re-install the 4 rubber bushings leaving the inner tubes out for now.
5. With the outer tubes cut off flush and the bushings re-installed measure the distance across the modified crossmember from the outside of one bushing to the outside of the opposite bushing - note this dimension - it should be the same as the distance between the brackets on the bottom of your car (2.75 inches).
6. Cut the inner tubes to this length, remove the bushings and install the inner tubes and bushings in the outer tubes.
If you are concerned about cutting too much, etc, you can take some measurements first:
1. The distance between the brackets on the bottom of your car should be 2.75 (2-3/4) inches.
2. The width of the crossmember without the extending tubes (as if cut off flush) on each side should be 2.25 (2-1/4) inches (measure before cutting).
3. The length of the rubber bushing extending beyond the tube on each side should be .25 (1/4) inch.
4. The modified crossmember would then be 2.25 +.25 + .25 = 2.75 wide., same as mounting brackets.
Ford changed the undercar mounting bracket dimension on Fox Mustangs from 2.75 to 4.50 inches starting in 1982, so 1982 and later Mustangs do not require any mods to the car or double hump unit to install it.
According to FMS catalog:
Fox cars with 2.75 brackets:
78-81 F/Z
79-81 Mustang
81-82 G/C
Fox cars with 4.50 brackets:
82 -83 F/Z
82-up Mustang
83 - up L/M
80-88 T-Bird
Good luck,
Paul
Please click here to check out my 78 Zephyr at my crappy Cardomain webpage
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