400-451 help

Any engine shop with a crank grinder and a competent operator can handle that. Any shop with a large enough lathe can handle turning down the counterweights.

Honestly, based on the jackassery I'm dealing with now with my 452, I'd open up the main journals to 2.750 (440) size just so you have bearing choices. There's only two full grooves bearings for the 383/400 journal, a Clevite plain and a Sealed Power plain. For the 440, you have Clevite P and H, probably V's too, plus King's, Sealed Power and probably some others. S/F....Ken M