I never did one. But i was thinking about an extended cab for a while. I had a 94 extended cab truck that would have worked pretty well. I suggest taking a tape measure out and start checking some wheelbases out. When you say, 63/4, what do you mean. The 63 cab is different than the 64 at the windshield.
I would imagine that 2 cabs would be the normal way to go. With the roof panel being the hardest part. However if you have a scrap 'burb around, I think that might be the easier way to get the roof right. Check the gallery for crewcabs and PM those guys.