Installed aux fuel tank in bed of truck, can I feed the 3/8" line from the aux tank pump into the Fass return line into the stock fuel tank. Or plumb it in how? The stock vent line?

 

I teed mine into the vent line to fill the stock tank.
Have an electric valve to shut it off when the stock tank is full.
Thought I would have to be careful not to overflow the tank if I spaced out closing the valve but it doesn't seem to matter, no overflow.

 

I have a ATI 30 gallon auxillary fuel tank in my truck bed under my Weathergard tool box and used an RDS install kit purchased from Northern Tools which is designed for our trucks. The kit installs in the fuel fill hose and includes a check-valve. Other places sell this type of install kit including eBay.

Mine was gravity fed per the above kit but later added a flow-through electric fuel pump in-line between the aux. fuel tank and fuel filler hose.

Link is listed below. Hope this helps.

http://www.northerntool.com/shop/too...5501_200315501

 

just gravity feed to the main tank, does not overflow. here is a pix of one that gravity feeds through a 2 micron filter before reaching the stock tank