what year service manual 73 or 74

I don't think there'll be much difference. Might be some difference electrically.

However, the newest one we have up for download---and it's free-- is 73, here

http://www.forabodiesonly.com/mopar/showthread.php?p=1970088617

There are two sections, body and chassis:

www.letsgocomputers.com/docs/1973dodgebody.pdf

www.letsgocomputers.com/docs/1973dodgechassis.pdf