Hidden rear tanks for the qMLS system were not added until just after the start of the '05 model year.
If you plan on adding extreme amounts of ballast, the 8.1L engine may actually be more effecient has it has more low end torque. The difference between any of the engines though won't be that much, and if you're buying an LSV, shouldn't be much of a consideration in the purchase process. The 383 was no available until the '05 model year.
For more information check out
http://www.MalibuBoatOwners.com