It would seem to me that two different kingpin inclination angles would have come about because the scrub radius is different between a typical 17" FED tire and a 15" Funny Car tire. I think you'd be best served to keep the intersect line of the kingpin near the center of the tire contact patch. So on a digger that may well be the smaller 7.5 angle. I'd mock both up on your existing axle to see what looks best.
Truth be told, either would probably be OK, but one is probably more optimum than the other.
I once was looking over my buddy's new altered. Apparently when the fabricator welded on the bat wings for the 4-bar front he did it with the axle upside down resulting in the kingpin bosses being canted in the opposite direction from normal. I pointed this out to my buddy and he insisted that it was supposed to be like that, even when I explained that the steering geometry was all catty-whumpus like that. He went on to win two IHRA season championships - so what do I know?
