No need to spend $$$$ for aftermarket mounts. Just use your original 273 mounting brackets and mounts. You'll need a few washers for a spacer one of the driver's side bolts (you'll figure out which one when you try bolting the bracket to the block, and either just use two bolts on the passenger side, or weld a triangle on the passenger side bracket and drill one hole. Viz:
View attachment 1715757082
Ignore the caption at the bottom that says this is a modification for 64-66 273 bracket. This is correct for 67-72.