[dsraven]

its good to decide what tire width you want, what wheel offset you want and draw a scale model of the frame width, tire width, box width, fender width from the inner lip of the fender opening roll etc. draw it like a cross section would show. you will quickly get an isea as to what you will need to do for a wheel tub or an axle width. don't forget to leave room for tire bulge and axle movement as you corner etc. if using leaf springs there will be less lateral movement but if using a suspension type with a panhard bar there will be lateral movement as the suspension travels up and down.
yor axle choice should be partially dictated by the amount of power you plan to have on tap as well as the amount of traction available to plant it down. it would be great to have the unlimited funding to buy whatever but if you plan to use what you have then I suggest you crack them open and have a look inside. make sure what you have is actually in good shape and will handle the power you plan on using. if you don't plan on narrowing the axle or having special brackets to contend with then slipping an axle you have under there could be saving you money and getting you driving and then the axle could be easily changed later if needed. the ford 8.8 is also a popular axle, is comparable in strength to the chevy 12 bolt and comes readily available with large axles, a posi, decent ratios and disc brakes with a top hat style park brake. the ford explorer uses them but has an offset center section. the mustang also uses them and has a centered middle. some will shorten the long side axle tube to be the same as the short side and use 2 short side axles. there is more info on that in "the ranger station" site if you're interested. I used one in a task force frame with the stock explorer width. redrilling the axles and discs to a chevy pattern is pretty straightforward as well, drill the chevy pattern between the forn studs.
if you are thinking of a new leaf spring rear suspension you could use a modern frame mount, possibly from a ford ranger which uses a 2.5" spring width, available from a number of suppliers. you could "mock" bolt that onto your frame as an assembly and adjust the height till you get it where you want it, with the weight of the finished truck sitting on top-after the front end is installed and completed etc. no special kit required and parts available from most wreckers of new stuff is relatively cheap and not a specially made item you gotta wait for delivery. just a thought. new spring pads can be had from a trailer axle supply shop or a speed shop online, to weld onto the axle of choice.