14 flat. Do you want hp guesses at the crank or the tires?
Seriously though, 400s can be decent engines, but in stock configuration were a bit of a dog. If you have a stock rebuild with a few bigger pieces, no bueno. To get better answers we need to know what KB pistons, deck height of the block, cc of the combustion chambers in the heads, port work - if any - on the heads, lift and duration of the purple shaft, cfm of the carb, stall of the convertor, type of tires, altitude you run it at. There are a lot of variables that can impact what the engine produces and how fast it will run a quarter mile.
Where I live, I've seen several 400hp recipes that put 290 to the wheels and run 15.5 quarters because the details of the build were not worked out well and the altitude kills their power and then they try to put it all down with a set of BFG hockey pucks.
