What did they (axles) come out of?

A-body 8 3/4's are 52 5/8" from housing flange to housing flange. So at 49.5" that housing is quite a bit shorter and will require shorter axles. A BBP axle for a stock width A-body 8 3/4 should be about 28" from flange to splined end. Yours would need to be about 26-7/16" for that housing (BBP). If you wanted to install SBP axles, they would need to be about 26-1/8" from flange to splined end.

Usually the BBP OE axles that can be re-splined are from C-bodies or trucks. But all the BBP axles have the same hub register diameter which is ~2.825". You can check that against the standard diameter for the bore on a BBP drum, Bendix advertises their BBP drum hub bore at 2.836". More Information for BENDIX PDR0182

Which brings me to the next point, if you're checking that rim against that axle without the drum installed on it you may not be getting an accurate idea of what will fit, because the drum will space the wheel out further from the axle register. It also looks like there's quite a bit of scale and paint on that axle register, so you may try cleaning it up with some sandpaper, installing the rim and then checking the wheel fit.

Changing from BBP to SBP is not as easy as just drilling new holes in the axle, as you're finding out. The axle register on the BBP axles are larger. The axle flange offset, the distance from the axle bearings to the face of the axle flange, is different (which is why the BBP and SBP axle lengths I listed above are different). Which means a different offset on the backing plates. So BBP and SBP brake stuff can not be interchanged.

If you want to use SBP axles with that housing, they'll need to be custom and roughly 26 1/8" long, but you should take your own measurements. You'll also need SBP brakes. If you're going to use those shortened BBP axles you'll need a BBP brake assembly. The BBP wheels you have should fit as long as the drums are installed.