Here's the one you're thinking of, Steve, done by R&R vacuumcraft.
This contrasts an untouched AMT cab and box side with the resin '72' casting. A lot of work was done to get the box back to a regular wheelbase configuration and he molded the bump all the way along the body. Other than that, the cab and wheel openings are unmodified. The chassis plate is actually a lengthened copy of the one from the '61-'63 AMT unibody model kit.
Here's what's included for grill (compared to stock AMT '76/'77 grill), tailgate and dash. I'm not trying to hammer the guy, but though a good start, the resin kit is basically unfinished and pretty inaccurate. If you like a major project, by all means find yourself one but I've been thinking of starting fresh on an extra cab I have to do a more accurate conversion. You can see the measurement notes I made on the sketch in the pic to help get the dimensions right. Might take a while but if any of you guys want to try whittling the various years of grills, dash or tailgate maybe we can get a master together for one of the resin casters to make. I'll be using a scale of 1mm = 1" -1/25.