400 Gross power?

Generally, you can add "in the neighborhood" of 100 horse power going from net to gross. One example is the small block 400 Chevy. The highest HP rating stock was 265 (gross) the lowest was 150 (net). There were several other versions made, but that's the best comparison with a same displacement engine between net and gross.

Another comparison is the Ford 400. The 1971 model year was 260 HP. Both the Chevy and Ford engines could be better comparisons, but IMO are the closest to compare to. The Chevy has a tiny bore size (4.125) compared to the Chrysler 400 (4.342) and a longer stroke (3.75) VS the Chrysler 400 of 3.375. The Ford has a 4.00 bore and a 4.00 stroke (really a 402) so neither engine is what I would call a "great comparison". I think it's safe to say the Chrysler 400 was somewhere a little north of both the Ford and the Chevy 400s, simply because of its huge bore size. Now torque, is another story. IMO the Ford wins out with its 4" stroke.

If I had to "guess" I'd say the 400 Chrysler was probably around 275 HP in gross trim.