I've read a bunch of posts, and this is the idea I came up with. Figured I'd make the drawing and have people look at it.

One thing I wasnt too sure about was the Aux return line that people have said to add. Is this only needed if the Aux tank cap isnt vented? I'm going to make a custom tank and use an aftermarket flat style fuel cell cap, vented.

think you might put valve(electric/manual) on your bottom line to fill tank thru vent line.

Just buy this kit from Northern Tool.

http://www.northerntool.com/webapp/wcs/stores/servlet/product_6970_200315503_200315503

so if the cap on the aux cell is vented I shoudnt need the top (orange) hose?

I've seen the northern kit...does it actually stop flow when the fuel level reaches it?

The biggest concern I have is when the fill pump is running I dont want to somehow over pressurize the main tank and pop something, thats why I was going with the orange vent/return line on the top of the aux tank.

I would recommend a return line into the aux tank when you use a pump to assist transfer. If you forget to shut off pump when stock tank if full the fuel will flow out the breathe or cap and across any thing behind you :D (from experience). Gives a piece of mind.