KC, The 307 code is L14. I'm not sure about your 3 spd. You can try here
http://www.rpocodes.com/ or here
http://outintheshop.com/options.html look in the m's like m15 through m19.
Sense your engine and tranny were standard they won't be on the SPID, I don't know about the firewall.
On the upper pass. side of the firewall was usually the model number, if that helps.
You did a great job on your 307 man, can't wait till you fire it up.
