There are a number of relay conversions available on the market to address this issue. Try this link for more information:

http://www.painlessperformance.com/webcatalog/largeview.php?SearchField=30815

The headlight relay conversion is also available at:

www.lmctrucks.com

The '60-'66 Chevy trucks are infamous for dimming/yellow headlights, under load.