Every brand I know of make the pistons under size to fit the over size bore, you have a bad set if they aren't smaller on the skirts than the 4.350 bore size  Send them back and get the correct size   Maybe they where made for a 4.355 bore size   A .030" over piston for 4.350" bore size should be 4.345" or smaller if it's a forged slug. Most will want at least .005" wall clearance. The clearance is "built in." If they are really 4.350", then they aren't for a .030" over 440.
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
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