I've been told any rubbing at all will burn them up. I have straight 3/8" Mantons in mine, .134" wall IIRC. They are HEAVY.
I did a TON of grinding on my KB block to get them to clear. The heads were ok.

CHIP
'70 hemicuda, 575" Hemi, 727, Dana 60
'69 road runner, 440-6, 18 spline 4 speed, Dana 60
'71 Demon, 340, low gear 904, 8.75
'73 Chrysler New Yorker, 440, 727, 8.75
'90 Chevy 454SS Silverado, 476" BBC, TH400, 14 bolt
'06 GMC 2500HD LBZ Duramax