If you can't find one that will work with 6 volts, you can hook a 12v light to a spare 12v battery and run it that way. I recently purchased one from AutoZone that shows RPM and has a timing advance button. I've been using it on my 56 which is 12v and it seems to work alright.
http://www.autozone.com/test-scan-an...ght/108152_0_0