If your engine is 100% stock then I wouldnt want to push more than 230-240bhp through it, the rods are your weak point.
Also if it's never been rebuilt then you will be down on compression (ring wear) and the head will not be breathing as well as when new (cam/lifter wear) so you may not make 250bhp but it wont be far off! With a good FMIC and mapping of course!