holy cow. your one lucky man!
Honestly I would try to keep it at 20ft that what makes it amazing. We have a 20x5 @ work and you just cant beat that length its amazing to watch the fish swim so freely.
Could you cut the tank in 2x 10ft parts then re seal it in the basement back into 20ft? its 20ft just amazing you can swim in that thing with the fish! think of that 15ft you wont be able to do the back strokes very well ;D
If it was me I would of got one of those cranes with a big bell thats gotta make a hole big enough within a few seconds