Congrats on the find, how do you find it being a longback?
I've an old DPM one, but it's a shorty. Worked great with beltkit etc, but now that's not required anymore it sits just a bit too high (like above my kidneys!) to be comfy! A longback was far too tall with beltkit on but I was thinking it might be a good choice for normal use nowadays.
They are pretty bombproof, and you've got to love the ability to stuff 120L in there, not that you have to of course. As for the floating lid, I wouldn't bother if I were you. I've a civvy Lowe Alpine rucksack that came with a floating lid to be able to use the snow-skirt under it to take it from 80L to 100L, and even that gets pretty ungainly and lopsided when having to do so. With 120L as-is I don't think you'd be very likely to really need any more capacity, and even if you did you'd probably find it a hassle to try and keep balanced.