Okay, misunderstood. Last time I fixed them on a B2, I used thick alloy grade 8 washers, about 3" wide and attached them to the rusted out bracket. Since the front body mount uses these:
Threaded in the center with bushingbr Upperbr
shop.broncograveyard.com
and these:
Threaded in the center with bushingbr Upperbr
shop.broncograveyard.com
which are a pain to duplicate, which I have done to make a custom mount, I would make my own steel frame brackets and just use a regular generic body mount from Summit Racing that takes a 1/2" bolt.
4x4 x 1/8 (11 gauge?) would be enough and when cutting the upside down L shape bracket, do it at an angle so it looks like a triangle from the side.