I'm thinking it's a possibility. Since the 40 and the T3 are the same width, you'll have the added structural support of two side pieces.

I'd recommend shooting Ali over at AP an email (or call, of course) - she's good people. See if you can learn what weight limit they recommend for setting something on top of it that doesn't exactly match its dimensions. I've seen other AP cages (of the same footprint) stacked relatively tall, but that's different since their weight is completely supported by the side pieces aligning with each other.