I believe it is going to be hard to make that much power, especially w/a siamesed stock type cyl head.
You could get 350 HP but I would think you need to up the compression more. But that wont make much of a daily driver w/that much compression of 14:1+ plus you will need a very radical camshft. 1.94" intake 1.60" exhaust.
Sissells (Mike Kirby) can get the pistons you need to fit the cyl head chamber.

Port work, larger valves lumps installed Larry. http://www.t6racing.org/

Mike Kirby, great cyl head work also. http://sissellautomotive.com/

IIRC, Sissells 250 engine w/the 12 port head 10:1 compression made 320 HP @ the crank. Very mild build BTW. But the induction set-up, pretty wild IMO, Blue engine.
Also, IIRC, here is the engine: http://sissellautomotive.com/completeengines.html

http://sissellautomotive.com/cylinderheadpics.html

Guys making 450 HP range 250 CI (Naturally aspirated) in Brazil are running a certain % of nitromethane from what I have been told.

IMO, you would be better off supercharging or turbocharging the 250.
Once you go w/a boosted engine you will never want to be naturally aspirated again. Two cents.

I think for an auto X track, a supercharger might be a better choice.


MBHD

12 port SDS EFI