Unless you plan to get radical I would expect around 400 horsepower.You can use 340 manifolds if you do some basic porting to the exposed areas but the critical thing is get a custom cam that has a wider lobe seperation and more exhaust timing and you would still be very close to 375-390 which is easy 12s.Beside some mods to the manifolds the big reason the stock class cars go so fast is cam timing,they have to have a mellow idle and push air through restrictive exhaust and the cam is the crucial piece to make that happen.