MS69, here are the engine frame bracket dimensions, measuring from where they mount to the frame up to the center of the cross hole for the motor mount:
Low perf. small block...drivers side: 2". passenger side: 2 3/16".
302...drivers side: 1 7/16", passenger side: 1 9/16".
Big Block...drivers side: 1 3/4", passenger side: 1 3/8".
Compared to the standard low perf sb brackets, the big block's are 1/4" lower on the drivers side and 13/16" lower on the passenger side. Since the drivers side sb bracket is 1/4" taller than the bb, that might be all you need, unless your car already has the 2" sb frame bracket. Also know that the mounting horn on a bb frame bracket is narrower than a sb, so a bb motor mount won't slip over a sb frame bracket.

 

To add to that, if you change engine mounting brackets you need the proper cross member to go along with that. BB or SB cross member respectively.

 

I have Hooker Comps on my 69 BB. I had to dimple my #3 tube at the steering box. I am also running solid mounts because it is real tight at the box.

I originally bought the Super Comps but one of the tubes (I forget which one) went over the top of the box so I couldn't get my lines in

 

I thought the bb mounts also moved the engine 1 inch forward in the frame.