Disc Brake LBP setup + 8 1/4 rear wanted
See if you have a U pull type scrap yard in your area. Best bang for the buck. Guess depends on what they pull in so best to call to see what years they service. Local one here has two lots. One for newer cars and one for the older. Inventory is computerized so you can call and ask if they have certain models. Really nice as they can tell you what they have, how long it has been there and where it is in the yard.
Watch Craigslist as well for later A bodies that have the parts you need and ask around to friends, work etc. as who knows what their neighbour might have in the back yard or they see every day on the side of the road on their way into work.
I did the same swap to lbp on a '71 Dart. I picked up a '73 parts car pretty cheaply by talking to some mates of mine. The one says "I think this guy at work has and old Dart he wants to sell." Little delay for title stuff before I bought it, but found a car in less than a week. I got the lbp disc brakes etc. off of it. The rear on it was a 7.5 so no use.
Went to the local U pull a few times and kept an eye out for a rear. Not too long and there was a '75 Scamp in there with an 8.25. It is a non-posi 3.21. Great shape. Pulled it myself right there as soon as I saw it. Ended up paying $100 or so for it. Rebuilt the brakes, checked the gears, changed the fluid, painted it and got it all swapped in (posted experiences with the swap on this board along with little things I found along the way in doing the swap, driveshaft shorten etc.) and been using it ever since.
Best of luck!
Cheers