Tom Brownell's book How To Restore Your Chevrolet Truck covers the conversion to some degree, but not really very in depth. Most of the conversions I've seen involve finding a 5 window doner cab and either swapping the entire cab, or cutting the 2 cabs apart and swapping the rear section of the 5 window with the cowl and A pillars of the 3 window.
Another option might be to check with the parts houses. Some are starting to carry the larger replacement panels from Dynacorn, inclusing the outer and inner panels for the 5 window. The body components are different, so it is more complicated than simply cutting the openings, as there are reinforcement strucktures in the inner stamping.
