That's right. You will have to go to a Mustang II front-end, which allows you to cut out the shock towers. A stroked 9.5" deck 351-based motor, using a 4.125" bore Dart or World block (even larger bores are possible) will allow you to achieve big block displacements in a much smaller package. For example a 4.125" bore, 4" stroke will yield a 427ci motor. Going the max bore on these will yield around 440ci.
While this is possible, you will have very little header clearance at the shock tower. One thing to consider would be a 9.2" deck Dart Sportsman block. Building on that platform is a little more restrictive, but has the advantage of fitting between the shock towers with room for 1 3/4" headers. Just a thougt.