by buying a different engine?
nah seriously the i6 has always hate big rpm due to the fact that the engine is undersqure design (stroke is larger than the bore)
so the whole rotating mass just explodes at high rpm
the only ford i6 that wouldnt mind being reved up so high would be the new ba falcon i6 engine.
best bet would to build up a 6000rpm limited i6 with just a set of rod bolts and preped/blue printed standard internals (save for pistons as some lighter ones will help the motor pick up engine speed quicker.)
and then spend the money other wise use to make the engine rev on doing the cylinder head and valve train and maybe even a efi set up.
tho if you do insist on revving the poor little 250
get a set of 200 cu in i6 connecting rods (ohc engines best to use au 6 inch rods) and some lite weight hyperuretic pistons to suit get some good rod bolts
have the crank cross drilled to lighten it
use some light weight valves and good roller rockers maybe some hollow push rods.
have the ports enlarged
and a MASSIVE CAM!
hope that helps.
live in S.E. queensland? bit of a revhead? click here!!--> speedphreaks
1000hp is underway. rods,pistons here!! t88 on its way!!