Hello,
I've known of folks dropping the but section from multi-piece rods to make a shorter rod. The only way I can think of to figure out the line weight accurately would be to use the Common Cents System. If you haven't heard of this system before, it is a way to determine a rods line weight and action. It really does work, as I've tested some of my rods using this system.
Here are some links for info on this system.
http://www.common-cents.info/
http://www.superbob.org/CC.htm

The second link is to the page with actual rod test data. I have used this for reference many time. There are tests on Dan Craft blanks there with the butt section dropped to make a shorter rod. Hope this helps you out a bit.

Jeremy