Remember 6-methyl-MDA is inactive, so the 6-spot may not be very tolerant to bulk. On the other hand MMDA-2 with a 6-methoxy group is active, and its N-methyl derivative too.
I would also be concerned about the toxicity of 6-chloro or 6-nitro MDMA or MDA. Why the 6-position anyway? The 5-position is more likely to be the sweet spot I'd say, 5-methyl-MDA is significantly more potent than MDA itself according to Nichols paper on it.