The headlight switch has a circut breaker built into it. There is no fuse per se. If the high beams are working then work backward from the light connector with a test lamp. If there is power and ground at the connector the bulbs might be the issue.
Suggest you start with a 70 factory service manual electrical schematic in section 12 and not try to use a generic. You can download the 70 manuals here.
http://67-72chevytrucks.com/vboard/s...d.php?t=558016