Should the Lockheed R6V Constitution have stayed on the board?

I don't think so. Interesting aircraft though, and the full double-decker design was of course finally incorporated in the Airbus 380. Because of the physical size of the Constitution (larger than a B767-200), in those days there were simply too many logistics problems as far as passenger facilities at airports, runways and ramps of inadequate strength, maintenance facilities unable to accommodate it, etc.

Not that all these things wouldn't have been resolved, since much larger aircraft eventually came along, but the airlines were simply not ready for that kind of technological leap, nor is it likely to have been profitable had adequate power been available. The Brits tried the wide-body concept with the Brabazon. Complete failure.

The problem with the concept of both designs was operating cost and, as with most things, it came down to the world of the "bean counters" and they couldn't get the numbers to pencil out. Had decent turboshaft or turboprop engines been available, then perhaps they might have been more viable, especially as military transports, but alas, too slow and the 707 would have put them out of business within a dozen years.