So I'd intended this craft as a stock cargo lifter, but didn't notice until I uploaded it to KerbalX that my Mk2-2.5m adapter was from MK2 expansion, and don't feel like fixing that here, so meh. I'm still proud of the design so I'm sharing it anyway.
Meet the Firefly.
Full album here
Designed for station component lifting, it comfortably puts 57t in LKO (Mk3 long rocket fuel tank used as test payload). Average efficiency flight profile leaves 600d/v in the tanks for orbital maneuvers and rendezvous.
Takes shameless advantage of magic autostrutting to keep the payload in place. Just pick the biggest central part of the cargo and select "autostrut heaviest part" and it'll lock into the engine pylons with no fuss.
I'll probably fix the adapters in the next couple days; just have to decide what I want to replace them with.