My 69 Camaro was minitubbed before I got it. The back seat still fitted fine. The shocks were moved inboard but the ride was terrible. It had a mono leaf set up with tramp bars etc. I finished up ripping out the mono leaf and putting in a Comp Eng. adjustable Ladder Link set up with coil overs. It was 100% better.
I could fit 28x14.5 x 15 tires on no problem without flaring the rear guards even with the mono leaf.