Dave,
also, you might want to take a look at the tank fill system I put together that I use on a regular basis and keep me supplied with oxy. You will have to register with GLDG though.
http://www.thegldg.com/forum/showthread.php?t=26729
The sytem is a high pressure system and is not the same as your planned system, but it never hurts to look at how other systems work.
FYI, I will also be putting together a system (using a thomas compressor) like Trey's and your planned system when I find the motivation (or it finds me).