turbojimmy,
I have a real question, I'm not trying to be a smart a$$. Why is the clutch fan irrelevant at hwy speeds? Mine cycles on long grades and there is a noticeable drop in temp when it kicks in. It is new and was put in just a few weeks ago, the old one froze up on us in Indiana. My MoHo is Chevy based 454 also.
On edit, read about the filler tube and possible new radiator. I had mine recored rather than replaced, it was cheaper and according to the radiator shop owner the tanks were brass and not aluminum or plastic which seems to be the way they are going. Anyway, recoring takes care of the filler tube location, mine is extended too.