I'm using R44XLS6 in my 478M which is one step hotter for 7500-8500 series trucks that came with this engine, though it's in my 68 3/4 ton 4wd pickup it should be fine, the heat range is just right for the 351E. I'm sure it could be cross linked to a step colder or hotter if need be. Funny thing is the plug is for a 90's 4 cylinder GM application and it has the correct 3/4 in reach for most GMC V6 application
