Needing some details
Take off the entire intake manifold and all that from the 99 motor. Get a 4-bbl Vortec Performer RPM, and a 4-bbl <->TBI adapter. Put your TB & distributor on it. Use your existing transmission if there's a 4WD vs 2WD issue; all your existing accessories and their brackets; AC, PS, alt, etc. Done.
Then go visit a truck board where people are still doing things with the old small block, which is what the 99 5.7 is. They'll be more able and willing to help with the details. This forum is for LS derivatives; the motors that replaced that old mid-50s thing, that came in 97-up Vettes, 98-up F-bodies, 99-up trucks EXCEPT 5.7 and 8.1, and various other vehicles such as GTO, G8, TBSS, CTS-V, SS, and so on.
Good luck,
Mark





