Yeah the six speed definitely would be a better option there for first considering you're looking at 3.76 vs 3.1 using that calculator that would mean theoretically that your first gear would only take you to 28mph
Given stop and go traffic I think it would definitely be nicer having that 6 speed. Either way sounds like something worth looking in to further.
Someone who actually has a three wheeler here should check clearance through the trans tunnel. Just how tight are things there in referenced to the pics I posted of the trans. If it could be done I'd imagine this would be a hell of a mod, with most of it just being your own labor as a 6 speed Miata transmission is pretty easy to come by still.
