What is the correct way to install my 100 gallon fuel tank to my 100 idi without touching the stock tanks Im doing away with? Can i just install and electric lift pump and run the return line to the top of the tank?

 

pretty much. just a couple details here...
the return line should be fitted with some kind of check valve in its tip like the factory did.
the electric pump MUST NOT be used to push fuel through the mechanical lift pump on the engine unless its wired to only run momentarily. otherwise, a lift pump failure will result in you pumping the crankcase full of fuel, and destroying your engine.

 

Would i use a check valve with a ball in it?

 

Would i put the check valve in the tank so its under fuel?

 

Its not really a check valve.... I'm not really sure how to describe it...
Its kinda like a rubber flap at the end of the return line that will hold the fuel in the line if there is no pressure. With a little bit of pressure, it will let the fuel push out into the tank. But, when the truck is off, it holds the fuel so it won't drain out and fill the system with air (thus causing hard starts).
Without a flappy thing, I think you'd want the return line at the bottom of the tank so its always under fuel and can't let air sneak back into the system. Hopefully someone else thats done this will chime in with some more definite answers.

 

the factory unit isn't exactly your average check valve, but it serves as one.

i've also heard of people installing a check valve in the return lines between the tee on the back of the engine and where it meets the hard line on the frame of the truck, with the intent of preventing air intrusion problems, running on the theory that the valve requires a slight pressure to open. i hear it works, but have no personal experience.

but regardless of the details, its necessary to have something there to keep it working right.

when i had my tank down a few months ago, i took this pic, which shows the factory check valve https://www.dropbox.com/s/83cqddszds...2012.46.29.jpg its the red tip on the left tube in the pic

 

Yah I wasn't really thinking right the other night, really exhausted at the time.

I think my point is its thats more like a really low pressure check valve than most.

 

if you want real snappy instal Im sure the make three tank valves.. will be spendy...or get a second tank switch, switch between Aux tanks,,may need a sender they sell then two..or use one from older ford tank,,in your new tank,,,thats a lot of fuel...

 

speedwrench, he mentioned he's doing away with the stock tanks, so while a 3-tank valve might be interesting, i don't think it fits his plans.
but since you got me thinking about it, it seems that would be quite the rare item. getting it to work electrically would be a bit challenging even, as the average TSV uses one circuit to slam it to either one side or the other, and it would require a middle stop or a second actuator to make it a 3 tank system.
having a second tank selector valve would be the easy way.

another aux-tank option thats often overlooked is to have your aux tank serve strictly as a transfer tank, with an e-pump just transferring fuel from it to your main tank.

but i guess we've strayed away from what the OP is looking for...