I'll take a stab and see how close I come. ON EDIT: Ok, I had to come back and correct myself!
I had the years off by one. The 454 LXI was '96-'97 so it's out of the picture.
1998 was the first years of the Vortec engines. In 1998-1999 there was the 454 Mag at 385 HP (rect port head and forged lower parts) and 7.4 MPI at 310 HP (large oval with truck MPI and cast lower parts. PS. want to buy what's left of my 7.4? )