I gave up 4" of headroom at the rear and 2" at the doors in order to have positive drainage to the driveway. The apron makes the transition back to grade acting as a "gutter" at the same time to channel run-off until I can redo the driveway.
I had to compromise and use 6'8 tall overhead doors, not a big deal, the original doors were that height. The family still SUV gets in with room to spare.
There was no stone fill under the original slab, it was just poured on earth, but I had to remove the other slab first (the one poured in 1938?) and that doubled my disposal cost and machine rental time, thankfully I did all this labor myself.
24 yards of 3/4" crushed limestone (12 per 2 car bay) went in before the new slabs were placed. My concrete crew did the final grading and were impressed with my eyeballed rough placing. They compacted all the stone, laid the 2" foam, 12 mil visquene and wire mesh. The slabs and foundation are now frost protected.
From years of being wet and dry, the original sill plates were rotted out along with the tails of the studs, Thankfully the brick veneer was holding up the structure! I repaired all of that and raised the height of the foundation by pouring a concrete "knee wall" to make sure the wood framing would never get wet or rot out again.