I had a 92 with the HO 4.3. I ran a 18.3 with mine. It had a cold air intake and Flowmaster muffler. That was it. My buddie had his 98 ZR2 Blazer and it ran a 18.5 with the same mods. His was faster in 0-60 though because of his 3.73 gears vs my 3.42s
my buddy has one just like yours except he has cold air and a glass pack at the 1/8th he ran a 11.47 at 3700 ft. above.
nhra accepts multiplying it by 1.55 to get the 1/4 mile but since its kinda heavy you should use 1.58 and that should get you in the ball park. this time was with full interior and a spare. we went mudding stright after. little thing is sick. and the glass pack now that we broke it doesnt sound like a v6 anywhere through the rpms