You could put a factory tach in that empty space and just wire it separately. Check out this site that "David in GA" just introduced me to. Sounds like he offers factory tachs. You must be aware that your instrumentation would not be correctly set-up though. in order to have the original appearing set up, you would need to purchase an entire guages cluster (one without the warning lights and without the large gas guage).
http://www.tachman.com/
http://www.tachman.com/
