70 gas tank retrofitting

Working with a 70 Dude with in-cab tank. Want to swap it out for a saddle tank or something else that will work. Any body have experience with this? Thanks-

My brother used a 80's Ramcharger tank in his 68 crewcab
also heard of using 60's Mustang tanks in rear of truck

I hung a mid-70's Chrysler tank in the spare tire area of my '71 with a "behind the plate" filler neck.

Anything that's fairly flat will fit well back there.