My charger, currently in mock up, has a T56, RMS front/rear, Moser D60, and V10. I'm running 18" wheels which fill the wheel well nicely. i have 5" of clearance at the bottom of the car (rockers).

Car is definitely lowered. I still have more than 3.5" of clearance between the axle and frame rails. Based on my shocks and spring rates i could lower it another 1.5" still with no significant issues.

I upgraded to a 4" Aluminum driveshaft and also pushed back and raised the V10/T56 a bit. As such, i completely cut out the transmission tunnel and raised it about 4".

All things being equal, going with larger wheels will increase ride height but won't affect the axle to frame rail distance. What you are finding out is that the larger wheels necessitate lowering the car more so that it doesn't look like a wagon... as such, your loosing that much needed space between axle and frame rails.