400+ is not hard to get out of a well-machined 340-360 if you're willing to run around 11:1 (sealed power flat tops) and a set of cleaned up E heads with a 245-ish @.050 solid cam, and Air gap and a 750 double pumper.

I'm essentially building that same combo for a 360 for my 2800 pound Rocky but instead with a 236/242 @.050 Hydraulic roller that should make at least 425.

With stock iron heads with only basic bowl and runner work you can probably get around 360-370 without spending a ton of green..to go past that you'll have to put enough int othe heads that you might as well get some eddies.

Shop around, I found a pair of N.I.B closed chamber Eddies and an RPM intake for $1100.00.

400 hp is only about 1.12 out of a 360, not too hard to get if the heads flow enough air and the bottom end doesn't have to fight itself with poor machining.