Hello, my 1988 K3500 with the 7.4,TH400 and 4.10 gears has a high altitude emission sticker on it. It was originaly sold from a dealership in Wyoming, I purchased the truck last year and brought it to the Midwest area which is much lower in elevation. The truck ran OK but acted lean on cold starts and engine temp always hovered around 210 at highway speeds and just generally seemed weak for a 7.4. I had the PROM ID'ed before ordering a PROM from Harris performance. The PROM info was that it was for a 1988 5.7, pretty strange. The PROM matched the ECM so I am for certain that it had not been changed. Anyway after replacing the PROM with one that I ordered from Harris, the trucks lean while cold, hot running issues are gone and it acts like a 7.4 should act.