gas tank questions

[old crewcab]

Hello all, I have have some questions for all of you square-body wizards out there. My son and I are diving into a project that I started on years ago (he started showing interest in the truck world, so it's GAME ON again).
Basic info: 89 1 ton dually crew cab (square), cut down to short-bed length, original rear-end replaced with narrower cab & chassis rear-end, and widened stepside shortbed to fit over tires once lowered.
After dumping both side tanks, I replaced them with a K-5 blazer tank at the rear, only to find that once I lower the truck it is going to rub on the the diff. cover.
Now for the ??'s. Has anybody out there used a tank from a 67-72 blazer? After searching a million gas tanks, the 67-72 tank mentioned above seemed to have somewhat appropriate dimensions.
Next question is, will the fuel sending unit be compatible with the fuel gauge, since we have entered a newer generation of trucks? Now, I know there are all kind of custom aftermarket possibilities out there, but I am trying to go for something that is budget friendly.
Also, trying to go with factory looking parts on this build, so it looks like something that was somewhat factory built.
I appreciate any thoughts or feedback that would be helpful on my quest. Thanks!!