68 barracuda spring re location kit to gas tank clearnce

The off set kit is enough to move your tire right up to the inside wall of the factory tub. Then you need to move it away about 3/8 inch for street horsing around. This leaves enough room on my 68 Barracuda, to run a 3" pipe between the spring and the tank, all the way to the back, both sides with no dinking around with the tank or it's positioning.
But, without narrowing the rear end, you will need custom backspace wheels. I run 295/50s on 15 x 10s no problem; all the horsing around I care to do.
I have run 325/50s on 15x10s in a straight line at WOT with no rubbing, but cornering normally. Those required a bit of hammer-massaging of the inner tubs in key areas; no big deal. And with my lowered body, I had to roll the lips, just a lil.

If you install the 3" offset kit, putting your springs into the frame rails, you will have other problems. Firstly your rear shocks will be about vertical, which will affect your street-handling, and secondly; HD shocks will want to rub on the front of the fuel tank. And you still have to re-engineer the rear shackle mounts as to their angles and range of motion.
With the shocks vertical, they offer no sway control, so then your system will depend on the springs, and their arch is likely to fubar that right up. So now you have to figure out how to mount a rear bar. If you don't control the sway, the tires will/may get into the fenders , the exact thing that you are trying to avoid. So now you are practically forced to mini-tub it and narrow the rear end. Going with the in-frame offset, is sort of an all-or-nothing deal.