Hey Steve. I commend you for the exhaustive descriptions, explainations and theories about fuel tanks in the '60-'66 Chevy/GMC trucks. As with most of the aftermarket offerings for these trucks, your comment, "There is an "easy" kit available at LMC Truck and probably other places.
http://www.lmctruck.com/icatalog/cbe/full.aspx?Page=37 It's $200 and doesn't look difficult. Essentially, it's a 21-gallon tank, which I believe is the same as what came on some old Chevy Blazers.", is a pretty broad-brush assertion of the product, especially, since you haven't installed one in a '60-'66 Chevy/GMC truck. In most cases, the instructions, reagardless of how comprehensive, leave out some of the details, thus requiring some ingenuity and creativity, during the installation process. The only way to exercise these demons, is to do the install.
All of the '60-'66 Chevy 1/2 ton trucks and Suburbans and 3/4 trucks had coils/trailing arms in the rear, so a between the rails/forward of the rear axle is not possible.
We concentrate, exclusively, on the '60-'66 Chevy/GMC trucks, Panels & Carryalls/Suburbans, and I spend a tremendous amount of time and care helping individuals navigate the pit-falls of modifying, upgrading, repairing and enhancing these vehicles. I am only able to do so, with the years of experience wrenching on them. I would never attempt to squelch the ambition of a fellow truck lover, but in most cases, there is more to the process, than the 30-minute segment on "Trucks", with respect to tools, knowledge, expertise, money, time, experience and space.
I have found the best proving-ground for this excersise, it to purchase a truck and undertake the various modifications, then share your personal experience.
