Do you HAVE the 6.5 PV that it likely came with? If so, my recommendation is to try that first and see if you can tune to it. Usually, engines with low vacuum from large camshafts benefit from cruise......although that's not written in stone. So my recommendation is to try with a 6.5, then if you cannot get it like you want it, maybe look at a 12.5. Are you 100% SURE of that 18hg at cruise? That's important.