I wouldn't do it. You'd want a lot of water to float that much boat down but if you had that much water you'd probably have problems with the bridges at the end of the run. Someone with more experience rafting on the Lower Blue may be able to give some specific beta on what flows you would need for an 18'er but I would say you'd want at least a grand in there. I've never seen anyone take that big of a boat down there. I know people who've made it down in a 16'er at 850 but said it was very difficult. Ideally, your looking at something under 14', in my opinion.
"The world would be a better place if everyone kayaked."-Brad Ludden (Valhalla)
"You only get one chance to run a drop blind."-DD