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

December last year I took my Dart back from a shop after the bill went crazy. It would only run hot wired. Fast forward for now and I need to replace my 7 1/4 with an 8 3/4. I saw some online for 4K+(if only I had more money). Motor is stock, 350hp assumed but would like to have 500hp or so later. Buy one and have it rebuilt? Costs? Is remanufactured worth it? Just for street but plan to put my foot in it a lot. 3.55 or 323? Thanks in advance.image000000 (6).jpg