yes only 2wd in 63-72 years and both 4x4 and 4x2 in 73-87 are all 42.5" spring pad center spacing so you can use any of those 12 and 10 bolt rears all those years, you will have to relocate one shokc mount is all and maybe replumb the brake hose connection on the diff cover bolt bracket, and then youll be good to go
for 64-70 trucks you can convert to 5 lug disc brakes on the 2wds by taking the front either entire cradle crossmember assembly or the spindles rotors calipers from a 71-87 2wd truck, and from vans you can take the upper arms and spindles rotors calipers but not the lower arms the lower arm shafts on vans are way longer cuz the chassis is wider on vans by around 3" van sway bars are longer too cuz of this so you cant use van sway bar setup on a truck chassis either.
if you need ot know moree then lemme or kennyw know we can let you know what you can or cant use to convert a drum front truck to 5 lug discs, youll have to get hoses to match the year of calipers you are using too and reroute all the front brake lines too and use a prop combo valve assembly off a 71-87 truck and you can use the stock master cylinder setup from a 67-70 drum truck even thoguh some say you cant or shouldnt, yes the 71-up front brake reservoir is larger than rear but if you are using up all the fluid in the front reservoir of a drum/drum cylinder then you have something wrong anyways.,
i personally reccomend getting stock spindles rotors calipers off a 71-up truck rather than using those pre-71 disc brake caliper conversion brackets for the 70 older spindles,
also you can buy 6 lug 2wd rotors for the pre 71 trucks but they are kinda pricey, and then you still have to buy the caliper bracket mounting kit and then so forth.
the entire engine suspension cradle crossmember assembly ont eh front of the GM trucvks is held directyl to the frame rails with 12 bolts, its not that bad of a deal to unbolt and roll a 71-up cradle under, ive doen it a few times but youll have to remove the engine or hold it up from above somehow while you have the truck up off the ground, the cradle itself is the same thing all years, and doing it all this way saves you from having to disassemble the suspension components and having to deal with realignment problems and coil spring force and so forth.
the only little things different starting in 73 for sbc and straight six engine equipped models is that the engine frame mounting brackets changed shape and the mounting holes changed locations in 73, thats no biggie at all, just drill new mounting holes with a 3/8" steel bit and use your 3/8" hardware just like the factory did, it is easy as cake and then you can use 73-up mounts or 63-72 ones, whichever you prefer or have available, except that the two designs dont have tjhe engine sit at the same height(vertically)
and if you want extra beef directyl inside the frame rails directly under the frame engien mounting brackets then find a 3/4 or 1 ton truck 73-up(frame rails front third of length of truck are same height and width as 1/2 and 3/4 ton, but are 3" taller under the cab and front of bed areas) and rob the brace brackets from the rails, it keeps the rails from flexing from engine work load torque, i mifght have a pair around here still, or i might have tossed them all away by now, i had a few pairs from a 76 C-20 and a 74 C-20, and etc they fit the 1/2 ton frames, it makes a difference, just liek steering box frame brace brackets do, frames flex and twist, more than you think, but are deisgned to under the bed and cab areas, but you dont want it to up front however.
good luck
