8.75 Brakes

For a streeter, the SBP is fine.
To fit the widest tires you can into a Dart wheelwell, you will need a custom offset wheel, so you might as well stick with the SBP and buy a custom 15"wheel.
If you upgrade to an 8.75 with an LBP , your track will change, and you will need new custom offset wheels, again.
So, IMO, make your decision once, make it today.
IMO, for a streeter Dart, the BBP is a luxury you don't really need, no matter what engine is under the hood; the sbp is not your weakest link.

Having said that, if you have your heart set on a certain wheel that only comes in a bbp, well there is no good argument against that; I did it to myself,lol.
To use the bbp, I had to get get a C-body rear, and narrow it (and shorten the axles) to my specs. This got me the correct backing plates and matching drums, as well as the 7290 yoke.
My other option was to redrill the axles and the drums, but as you can see in your picture, I didn't think much about that. and the end result is that your bbp wheels will not center on, and be supported by, the hub of the sbp axles.
If you order custom axles,
#1) the flanges will be further out, making your track wider. No big deal except your shoes will end up not inside the drums. So
#2) you will have to either get different backing plates or fabricate spacers to push your backing plates back inside the drums, and
#3) you still have to deal with the larger wheel-register not fitting into your drums. and
#4) now you are stuck buying a new set of custom offset wheels,...... again.

Hyup I learned the hard way. That was 2004 I think, long before I heard of FABO. Long before I even had a computer,lol.