For gmt 400 with Hydroboost you either run a stock 88-98 Hydro. M.C., or you can bore out the inside of the hydro unit to accept the NBS Hydroboost M.C. I have done both, I have the NBS Hydro. M.C. on my personal truck, I used a M.C. from an 04 tahoe with hydroboost, then I bored out my...