Need an 8 3/4 for my 69 Dart 413/727, help?

4K seems pretty steep no matter where you live. You can order them new for less, built to your specs.
Are you staying 5x4 bolt circle, or going to 5x4.5? Drum or disc brakes? Type of limited slip unit and gears? It all adds up, and if you drop 2K+ on a stock rear and then start replacing parts, it adds up fast. Bearings, gaskets, gears, brake hardware, axles if you're changing patterns... Plus don't forget the additional ancillary items- U bolts and shock plates (different from your 7.25 stuff), a new driveshaft (shorter than the one for your 7.25), and probably leaf springs... etc. etc.
For pricing on new stuff, check Dr. Diff and Moser engineering for starters:
Mopar A-Body 8 3/4" Housing & Axle Package (doctordiff.com)
8 3/4 Mopar Stamped Housing & Axle Package - Moser Engineering
By the time you add up the cost of buying a used rear that needs rebuilding, buying all necessary parts, rebuilding or paying someone to rebuild it, buying new right off the bat starts looking a lot more attractive.
Or a nice rebuilt one from a member, of course!