for reference it is a t56 behind an ls1, I don't have the transmission apart so i can not take measurements, my best guess would be just use an extended pilot bushing in the crank. But I am not sure if that leaves the splines fully engaged on my twin disk.
If the plate is only 1/8" thick, then I would likely not worry too much about pilot engagement. Or remove the stock plate and use only a midplate, then you lose nothing. Does the new cars even have a factory shield between motor and flywheel?
On my car, I have two plates for the fact I needed more room inside the bellhousing. I then used the stock one to weld ears on to make a midplate. The second ring is just that, an open ring so it fits around the flywheel and just spaces the bell outward. There is not enough room behind the flywheel for a full two plates.
The stock ones don't have a shield but my sfi bellhousing does use one. I was going to use an alum midplate, but a steel 1/8th and a thick front alum should get the job done. Do you use anything to control forward/backward motion of the engine or have the plates themselves been sufficient?
When I ran a Jerico my midplate was made from the same steel used for armor. 0.250" thick. Was a real bastard to work with, but I had no worries about the clutch destroying the block in the event of an explosion.
I made a limiter for the right side. Right and left hand heims, mechanical tubing, etc...