if your going plastic, be aware that the rmr crew model is expensive (w/ship almost $65) & you still need a donor kit, the AMT '84 GMC would be your best one, my crew model, donor kit, & misc junk cost me close to $200 (for a model??!!)
if you want to do it reasonable, i would buy 2 chev trucks & make your own longbox. i'm working on a el camino right now, but i do have a spare shortbox that i will be turning into a longbox once i'm done.
it's not really hard at all
you will also need to stretch the chassis 12mm behind the cab, lengthen your driveshaft & your exhaust (evergreen styrene 3/32 or 1/8 tube works perfect for both). the MPC kits have the best box for this, there very precise & even, as do the AMT's. revell 73-87 trucks really leave a lot to be desired