fouled plugs

A 400 cubic inch daily driver with that big of a solid flat tappet cam (252-256 @ .050) will not be very pleasant. It is also doubtful that the 906 heads in a stage one configuration will support 500 HP in a 400.

Here is a 440 dyno run on line #25 with stock heads in about a Stage 2.23343 port job with stock size valves. Notice it doesn't crack 500/500.

http://www.forabodiesonly.com/mopar/showthread.php?t=261378

I too am thinking the Muscle Motors is the better choice of the two choices you are asking about.