Well,
AT is hands down the best system currently produced by
GW for
40k related things and one might just go with that... Doing a disservice to yourself by using their
40k rules, which are pretty lacklustre for things on that scale.
The new Apocalypse is a better starting point, it should work reasonably well with the default ranges in inches or centimeters. I've been thinking about trying
Apoc with 6
mm models and given how much less space they take on the board, the default ranges work without major hitches (hell, normal infantry moved 15
cm / 6" in Epic and could do that thrice per activation

).