For a stock 250 I would just grab a turbo from a T-bird Turbo Coupe. Yeah, it's too small to make much boost, and it will run out of breath early, but an otherwise stock 250 is all done at 4200 rpm anyway. The little turbo will spool up fast with little lag and will easily give you 6-8 psi. That's enough to add 50-60 hp without risking the engine at all.