Kev,
When I set up my aux tank I used these quick couplers (Parker NS series) and I think they are 3/8", but I would probably go 1/2" to reduce restriction if I were to do it again:
http://www.parker.com/quickcouplings/Hydraulic/NS_Series.PDF
These fittings are nice in that both sides seal and have planar mating surfaces so that there is no spillage in your bed when connecting or disconnecting. I have seen some that are cheaper (in Northern catalog) but I cannot speak of their quality. I think these ran me around $5 for the males and $15 for the females. Technically speaking, it is better to use the Viton seals for fuel use, but I have had no problems with the standard seals in my fittings. More slick Parker fittings can be seen here:
http://www.parker.com/quickcouplings/
My system uses the Northern Hydraulic portable diesel pump with a couple slight mods (remove handle and hardwire):
http://www.northerntool.com/webapp/...PHOTOS=on&TEST=Y&productId=47671&categoryId=0
It is safe for diesel and pumps around 5gpm. The system runs one 1/2" fuel line connected to a nipple welded low into the filler neck.
Pics of the hardware can be seen here:
https://www.turbodieselregister.com/forums/showthread.php?s=&threadid=67265
I built a fuel sending unit into my tank that I purchased from JCWhitney. The sending unit is made by Datcon (I believe) and is the 240 ohm type. I believe this is the same type sender Chrysler uses, but I have not hooked mine to the instrument cluster. This type of sender would be relatively easy to retrofit into a tank.
One major redesign I am considering is running the system with a switching valve. I have already eliminated the front mount transfer pump and put a carter 15psi pump right by the primary tank and am considering replacing the Northern pump on the aux tank with another Carter. I will then have to plumb a second line through the bed for fuel return. This would then yield a redundant fueling system incase one of the transfer pumps failed--just switch to the other tank/pump combo. I have a Pollak P/N 42-300P (JC Whitney) switching valve that will facilitate this.
I had considered the electronic fuel level monitoring/switching approach, but was never able to find an IC that would work. That is one of the reasons I am considering the switching valve. Sorry for the long reply, but I hope some of this helps.
Chris