Wow, that's a huge grain bill! I only have a 5 gal round igloo for my MLT, but could always scale back the grain to accommodate my MLT, and add extract after (batch) sparging to get the gravity up where it needs to be.
With the smaller MLT that I have, I had resigned myself to brewing lower grav. beers. Do you think there would be any drawbacks to brewing it as I've described, kinda like a large partial mash? I think (since I've never tried it) I would max out the my MLT with 12# of grain roughly, based on what I've read here.
Also, have you tried your recipe yet? How did it come out? Any tasting notes?
Thanks for the response and the others as well.