Some good info posted. I built a low deck 451 in a 400 block back in 2001. I liked it. The main bearing journals on a 440 crank are 2.750" (nominal), and the 400 block used 2.625" (nominal).

In general, cutting the counter weights down to 7.25" diameter will clear the block. The bigger the bevel on the counterweights, like CompetitionWedge posted, the larger the counterweight diameter can be. A typical factory 440 steel crank has counterweights about 7.50-7.55" dia.

This picture shows where the 400 block interferes with the factory 440 counterweights. You can grind clearance here as an alternative.