Call Indy Cylinder Head, or Ray Barton. They build them all the time based on an R3 Block with a 9.6 deck and siamesed block that will easily go 4.25 on the bore. With an, as delivered, 4.22 bore, you'll get 476 cubes.
4.25 x 4.25 gets you...482 cubic inches, from a Mopar small block. All it takes is money.
Here's a good place to start: Mancini sells the blocks.
Cast Iron "R3" Race Engine Block Siamese
deck: 9.600" cyl. design: siamese over bore: 4.220" cap design: 4 bolt tappet angle: 48 journal size: 318/340
$2,595.00
MOPP4876673AD
