Be proud of your 5X4s especially if you have left hand treads on the left side! anyone can have 5 x 4 1/2 and right hand threads!
While I don't usually buy from rock auto (I just need the instant gratification) they do have A LOT of parts. The below link is for a 69 Dart with a 318, but a kit is available.
1969 DODGE DART 5.2L 318cid V8 Drum Brake Hardware Kit | RockAuto
Classic industries, Year One, Ebay and a ton of online re-sellers stock parts for our cars. the first step is to get the part number. Heck your local auto parts store can usually get many of these parts too.
Depending on what/how you get/put together your 8 3/4, your 5x4 10 x 1-3/4 brakes will swap over.
Disks up front, there is more to it than adding the disks, master cylinder, proportioning valve etc also need to be changed. possible suspension parts depending on the swap you do.
Lastly unless you are going to me making GOBBS of power any of the center sections will do fine. Your choice should be more driven by what gear ratio you want to run (I'm referring to new gears that I have found) 741 is limited to 2 or so ratios, 742 has 5 or more, 489 a few more.
As to new drums... 10x1-3/4 (I am assuming that is what size you have) are not as plentiful as other sizes BUT they are available. If your drums can be turned within safe specs do that. Unless this will be your daily driver they will probably last you a decade or more. 2017 I put 1000 miles on my dart 2018 600 miles and I drive it monthly.
Online reference materials:
MyMopar - Mopar Forums & Information - MyMopar Tools/Reference
As corny as these videos are they are very useful.
MyMopar - Mopar Forums & Information - Browse MTSC by Topic
1969 Parts book (PDF download)
http://www.mymopar.com/downloads/partscatalogs/68_Mopar_Parts_Catalog.zip
1969 Factory Service Manual (FSM) (PDF download)
http://www.mymopar.com/downloads/servicemanuals/1969_Charger_Coronet_Dart_Service_Manual.zip