Hello, Upgrading a 1971 904 that is behind a 350+ HP /6. Bored 0.060, milled 0.020 off block and 0.100 off head. Back yard port job, MPFI, and twin 2.0L turbos. Stock cam (not too bad in that year)

I milled the pressure plate on the front clutch set 0.050 to accomodate 4 race clutches in place of the 3 that were stock. I have 0.070 clearance there and think I'm OK. I would like to upgrade the rear clutch drum as well to accomodate 4 frictions in place of the 3 stock but I would have to mill 0.125 off the pressure plate. That seems like a bit much. I went to my bone yard and found out that I cant just put a 4 disk rear drum in as that would require new shaft and pump assy. Does anyone make a thinner/stronger pressure plate? Can I mill that much off w/o major warpage issues? Is it a big deal to leave 3 clutches in the rear drum? Some say its about line pressure not # of clutches, but I'm not so sure. Looking for advice!

Thanks,