That's a really good idea, if nothing else will add a helpful piece of information.Would you happen to have on hand another sizing die based on the 30/06 case like 25/06 or 270 or 30/06. If yes then try running your case all the way up in one of these dies till the base of the die is in sloid contact with the shell holder. This should size the .200 line and not affect your shoulder bump . Then use your 308 die to bump the shoulder. Might work.
Plan at the moment is to load the sized cases and see how they shoot with the bases sized and the shoulders where they are as a point of reference. I will also keep an eye out for a longer FLS die as suggested- thanks again.