Not sure if this is what you mean or not, but I put a 9.0 LaRue rail on a 16" barrel.  To do it I used the LaRue low-profile gas block in place of the standard FSB (Front Sight Base).  The front sight is now a Troy Ind. flip-up batle sight mounted to the rail.

If you are wanting to keep the standard FSB, there are a few rails (like the DD 12.0 FSP) that accomodate what you want.  

A Dissy setup has a longer sight radius than a 16" carbine.  In other words, the FSB is farther forward on the barrel than a carbine's FSB.  Make sense?  

So, you can achieve a longer sight radius with something like I did.

Hope this helps.

This is a terrible picture, but it should give you an idea of my setup.