changing bolt pattern

Find front disc parts from a 73-76 A-body. (Duster, Dart, Swinger, etc.) This gives you a 5 on 4-1/2" bolt pattern with 1/2" studs. Go to the Mopar Action magazine web site and look at the Disc-o-tech article as it outlines a lot of disc swaps using factory-based parts. I still think using factory parts is the most economical if you shop carefully but there are several after market disc kits out there.
You'll need the upper control arms, knuckles (spindles), calipers, rotors, upper and probably the lower ball joints, etc. Also a disc brake master cylinder if you have drums now. You can piece this together from different sources and you don't have to buy everything at once. get everything together before you make the swap. Since you'll have most of the front suspension apart, check all the wearable components and replace as necessary. Things like lower control arm and strut rod bushings. I'd recommend replacing the upper control arm bushings with the Moog offset problem solvers. You install them in the UCA offset from one another to gain positive castor. With modern radials align to positive 2* castor or so, a half degree negative camber, and 1/16"-1/8" toe-in.

For the rear the easiest method is to get a complete 8-1/4" rear from a 73-76 A-body. This would be a direct remove and replace swap. Depending on what you have back there now, you may have to adjust your drive shaft length. If you have a /6 or a std V-8 with automatic it will be a 7-1/4" rear. You will also need different shock plates if you currently have a 7-1/4.

As mentioned, you can have your SBP 8-3/4" axles and drums re-drilled for the LBP.

If your car already has an a-body 8-3/4" rear you can use shortened C-body axles or brand new axles from Moser, Yukon, etc. You will need new rear brakes complete with the matching backing plates. I used shortened c-body shafts and b-body 10" x 2-1/2" drum brakes but it's your choice.

A complete B-body 8-3/4" rear can be swapped in but you'll have to cut and re-weld the spring perches to match the a-body leaf spring width. Some people just spread the springs apart to fit but I wouldn't do that if it was my car. The b-body track will be wider than the a-body so you'll have to be aware of that when buying wheels and tires. B-body track widths vary depending on the year.

Doesn't sound like much expense but when you factor in new wheels and probably tires, it can add up.