I've read some posts about the miata 6 speed tranny fitting into our tranny cases. Rob from rotary shack had some information on this as well.

 

If you're n/a just put a miata tranny in there. closer ratios than 1st gen and stronger too.

I have used 626 boxes and they have even closer ratios, but the gears are physically 1mm narrower. Not as strong but my bridgeport has not broken one yet. friend used one on his gt-3 rx7 with fatty slicks and only thing ever broke was the bellhousing.

 

Anyone not considering the TII trans due to weight,they arent that heavy.And their placement (in the middle of the car) means that the few extra lbs arent going to affect handling that much.If youve got enough power to NEED a TII trans,then a couple xtra pounds aint gonna hurt.
The TII 5-speeds are extremely easy to find and are cheap, considering their torque holding capabilites.The ratios may not be ideal for all N/A applications,but its nothing a set of rearend gears cant help.

 

rotaryshacks new product opens a door....

http://www.rotaryshack.com/Products/Transmissions.asp

now i know next to nothing about tranny's, but from what i understand...
mustang tranny=good