Correct, the A-body 8 3/4 with SBP axles and brakes is narrower than the same housing with LBP axles and brakes because of the difference in the axle flange offset on the axles and the brake width.
Also, for that reason you
cannot use your SBP brakes with standard LBP axles. The D60 axles all have the LBP axle flange offset, so re-drilling the SBP brakes won't work because the backing plate offset for the SBP brakes will be wrong (it's based on the SBP axle flange offset)
I suppose if you really wanted you could have custom axles made with the SBP flange offset, but the larger bolt pattern, like they make for the 8 3/4's. But then you'd be forever stuck with re-drilling SBP drums. Which I really don't understand, because if you're already going through the expense of changing the axles and the wheels you might as well just do the better thing and use the LBP brakes too, instead of buying a weird set of custom axles to reuse a set of brakes that it keeps getting harder to find parts for.
If you have to buy a complete set of BBP drum brakes, buying a set of rear disks isn't a huge added expense. DoctorDiff's rear disk kits start around $600, there's a member here selling the loaded, rebuilt BBP backing plates for $300 but I don't think that includes the drums and honestly that's still a pretty good deal because usually just a complete, used set of loaded BBP backing plates is a couple hundred bucks now.
And yes, running rear disks on the street is a worthwhile investment because there's a really good chance it will improve both your braking control AND shorten your stopping distance.
Mopar Muscle did a rear disk conversion on a '73 Dart Sport and compared the stopping distances between the rear drums and rear disks from 60-0. Their result was that from 60 mph with factory disks up front and factory drums in the back the car took 133 feet, 6 inches to stop. After the rear disk conversion, the stopping distance improved to 122 feet 4 inches. It wasn't a super scientific way to test it, but it was better than most of the documentation out there. And the car involved was set up the same way as a lot of these cars, with larger rear wheels than fronts.
The online article is a bit of a mess now, probably something with being moved from MM, to Hot Rod, to MotorTrend after each of the previous magazines was bought out. The final distance is a caption on the second to last picture if you open up the images for the article. The hardcopy article is easier to follow, but that's the way it goes.
https://www.motortrend.com/articles.../?galleryimageid=274576?galleryimageid=274576
If you do use rear disks, keep in mind again that they too will widen the track width. Most kits widen the track around a 3/16" per side, because the rotors are thicker where they sit on the axle flanges vs the drums they're replacing. So you'd want to make that decision before you have the axle housing width set.
What wheels are you planning on running? Sounds like you already have the width and backspace set? And you have 1/2" offset springs?