Get some measurements. You need the length, so measure the bosses on the bell-housing and add about 12 mm (1/2 inch). If you are a little long it is ok (provided it does not bottom out) because the rule it that you want at least one diameters worth of engagement.
The bolts if I remember right take a 17mm wrench, and that should be a 12mm bolt. I think course thread. These had a flared area underneath the bolt head to help distribute the stress, a washer under them will do the same thing. But using a washer makes the bolts more likely to work loose.
Flying Miata salvage may be able to put together a set for you so check them out. These are not listed but if you have the time call them and find out.