B/c it's so easy to put a big stroker crank in one. That's the only reason I know of.  Seriously though...there isn't a reason really. Just have it balanced for the heavier 400 slugs, use the internally balanced damper and flexplate, and should be good to go.
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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