Bill, sounds like you are on to another good idea there. Man, I wish I could tell you what the inner diameter of the Ken-Co sleeve is, so we could calculate the air gap. Anyway I'll measure it when the new kit arrives, and post a reply to this topic.
And just like Ranger, I have had the .5" Ken-Co tube installed on the '99 now for a long time, with absolutely no ill effects either. (I have to wonder if, from GM'* standpoint, there may be a slight difference in emmissions if the tube is smaller than their .625?? Who knows.
But on the other hand, I seem to remember about a year ago someone actually checked into the "internals" of the EGR valve itself. Seems like they found out that the internal passages within the valve are actually much smaller yet. So if that is the case, I guess the .5 tube is no sweat. -(just thinking out loud, again,,