Bob,
I am at work so I can't check at the moment, but I think the filler neck on my (cab11) C8 is longer than the one on your cab11 rad(s). I have no idea if my rad is the original one or if it has been replaced with one from a cab 12....nor do I remember if the filler neck is tilted. I will try to have a look tomorrow morning.......tonight is a no-go, as we have already moved to those days of the year that it is dark when you get home from work
Almost sounds like the problem with the filler neck and cap was a known early cab 11 problem, which was solved during production with a longer filler neck???? Than again, it could also be a result of the mods on your truck by the previous owner as you described.
Alex