4" to 4.5" Bolt Pattern on a 7 1/4" Rear?

Yes that is definately a 7 1/4" rear. No you can not take axles out of a BBP 7 1/4 and put them into your SBP 7 1/4. The axles have a different taper and different bearings.
you need to look for an A-body after 72 with disks and a V8(preferably with a towing package) to find an 8 1/4. That is your best option for getting BBP.