So the extra 3/4 inch on each side of the B Body rear will put me close to where I am now minus a 1/4 inch of width on both sides but will have 1/2 inch more to the spring. (Am I not calculating this right???)
My BBP 7.25" rear was 56-5/8" drum to drum. A 68-70 B rear is 60 1/8". Even with 1" spacers, you're at 58 5/8". So, 60.125"- 56.625 =1.5" , divide by two and you get .75". So, the B body rear is going to be .75" wider, per side, than your current set up with the spacers.
I haven't measured the width on a SBP 7.25 though.