Tav,
On the 3rd gen B-body bracket, you are correct. The 2 outer holes are in the exact same spot as the E-body bracket.
Also,,,,The E-body bracket is actually maybe 3/4" of an inch longer on the upper plate portion of the bracket. So modifying it for use on an E-body would be super simple, although I personally don't think it's necassary.

When I removed the E-bracket off the car I parted out, I don't remember taking any nuts off from the bottom( several years back so anything is possibile )

The 71-72 B-body bracket was just removed this past week. And it had studs sticking up thru the inner fender with captive washer nuts holding it on. Next time, I'll ck from beneath to see how it attaches