The 425 is an early big block, using a 3.000 inch main, the 403 is a late
small block, using 2.5 inch main.  Cutting a main 1/2 inch is not
acceptable.  However, supposedly one can put the 425 crank in an Olds 350
*diesel* block.  (Some trimming of the counterweights may be needed)
These blocks are rumored to go as far in the bore as
4.25 inches (sonic check, of course) and that'll get you something like
450 cubes.  I'd imagine pistons might be the sore spot there.