391 industrial was an FE Ford. These trucks used 318 poly, 361 and 413's with some "special" characteristics. I have one or more of each engine in service. The 318 will surprise you! No ramp truck (yet) but we have two D600's, a D500 and a W500. My D800 doesn't count because it is from the later marshmallow cab era.

One interesting note is that these engines use gear driven, reverse rotation cams. I have a set of NOS gears for a reman short that we scored but no cam. From what I remember the distributor turns the right way so there must have been a special intermediate shaft too?