Hey Jazz1 how did you get the tubular top rail of your tailgate?....weld on a piece of round tube? And if so did you have any trouble with warpage due to the fact that the sheet metal wanted to shrink more than the tube?...
I used the mustang tank, removed the rear crossmember and welded a new one above the tank. It's the way to go. I don't mind the shallow bed at all, go with what YOU think looks best.
Yes,,welded a piece of round stock. One of the tasks I gave #1 son when he started his boilermaker apprenticeship,,he also made my gas tank. I bent the tin and he welded the seams.. not a single leak.
Take a look at the bed in this thread. It works well I think http://www.jalopyjournal.com/forum/threads/41-truck-progress-update.434746/
Shallow floor worked out for me, I put the battery, and fuel pump under the deck with a trap doors, and the fuel tank was already under there, so the filler was a natural. Your depth looks like it would look fine i.m.h.o. Good luck.