[PanelDeland]

WELCOME!!!
Read thru some of the suspension threads,you will find a ton of info on the disc swap.I would probably go with dropped spindles on a 73^ front crossmember with disc and power brakes.Try to get the whole set-up from one truck,including pulleys and brackets for the PS,though Water pump length may be different.This will change you to 5 lug front but you may be able to use the 6 lug rotors from a vendor or you can swap in a 5 lug rear from a 71-72.
The PS gearbox will work if you dimple the frame.Do a search on that and figure out how you want to do it.
A Blazer/burb/Panel tank will work on a long bed fairly easily but there are mods.It does hang down a bit,there are other options that work.Searching any of these topics will turn up lots of threads.Watch out on a used tank,most have condensate rust and crap in them and may cost more to refurbish than a new tank.The burb blazer tank kits are available from vendors(under $200 IIRC).Good luck and enjoy the build.