An interesting thing about the cartoon scaling of terrain in 40K is that the terrain nominally made for 28mm actually works great at 6mm if you add a few scale-specific details like doors and ladders. For instance, the 28mm imperial buildings (and the old cityfight stuff) are comically small if you actually scale them off at 28mm. The basilica administratum works out to being like 25 feet high with a floor area of 500 sf. That's a damn row house.
Whereas if you use the same building for Epic/AT, it's hundreds of feet tall, with soaring columns and a door big enough to admit two tanks driving side by side. To my mind that fits the fluff a lot better, for instance the spires and walls you can see in >>66335543
. You just need to dress those pieces up a bit by cutting away obvious 28mm scale cues and adding 6mm ones. Same goes for Fortress of Redemption. At 28mm it's barely big enough to be called a watchtower, and on a 6mm table it's an intimidating edifice.