Slant 6 most mpg's

It's called camshaft "centerlining", "indexing", or "dialling in". It involves very carefully measuring to make sure the camshaft's position relative to the crankshaft is exactly what it should be—rather than just lining up the dots on the crank and cam sprockets and hoping for the best. There are lots of articles about it all over the web; a Google search will quickly bring you lots of reading.