anybody know if an rx7 13b rotary will mate to an mx5/eunos/miata 6 speed gear box?

no, I believe you need to modify the input shaft and basically fit the mx5 internals into an rx7 box. Its been done before have a search for it on www.rx7club.com

found this article there
http://www.rx7club.com/showthread.php?t=481920&page=1&pp=15&highlight=mx5
nles im missing something, it looks like it should be a mater of shortening the input shaft only....
i hope to use a 13b engine in my locost, with the mx5 hubs/supension/rear diff setup.

my wheels will probably end up with a smaller rolling radius than an rx7, meaning quicker acceleration but lower top speed, hence the need for the taller sixth gear.

please correct me if i'm wrong.

The 6-speed is totally unrelated to the MX5 or RX7 5-speeds. It is an Aisin unit - iirc the same box is used in most current Jap rwd cars and many GM cars (Solstice).

Check out the RX8 - it has an Aisin 6-speed, but slightly different ratios to MX5.