I've been seriously investigating an EFI for my '68 Z24. I feel the best option is Holley's new "Sniper" kit (up to 650 HP). You can't beat the Holley network; and I think the "Sniper" is up-to-date, relatively easy to install, and a good value ($1250 for the master kit, w/free shipping, no tax-at least to Ohio). But let's talk about your situation...it's similar to mine.
First of all, I have a Procharger (8 PSI), which ATI says will work fine with the Sniper...but there is some contradiction with Holley. My biggest challenge, though, occurs back at the gas tank...just like yours. The OE sender in our Chevys includes a gas return line system (I assume your '69 has one) that does not come anywhere near meeting the requirements for the Sniper's gas-return. So, Holley offers four solutions: 1)-get a new gas tank with an internal fuel pump, sender, and return system...but as your are finding out, it may not be available for some of our big Impalas; 2)-get an after-market float-sender that has room for installation of a return line inlet fitting/hose extension through its top plate...this would mean replacing the existing OE cartridge sender which I prefer over a float; 3)-drill a hole and install an inlet on the gas filler tube...but this would only work if you could take the tube off to do the fitting installation...our Chevys are one piece (at least, mine is); 4)-drill a hole and install a fitting/hose extension through the top of the gas tank...this would be my approach (if I do all this)...but it would mean cleaning out the tank ahead of the work (safety), and cleaning again (remove shavings). Holley's master kit does come with the necessary fittings/tubing to install in the tank or fill pipe (along with an external fuel pump, filters, supply hose, fittings, hardware, and engine fuel pump plate); but not any return tubing/hosing back to the engine...I was going to use my original supply line as a return line...plus necessary rubber hose.
Wow! I may have gotten carried away! But I wanted you to know what I have encountered so far. I'm beginning to think I don't need it. My big Holley carb and blower combination work just fine and I continue to get decent gas milage....and that gas-return system is going to be a challenge for me. Plus, I would have to provide my own design/fabricate/install of any external components near the gas tank. Now, you can get a return-line system (at extra cost, of course) that does not require any return line to the engine compartment...but you still have to provide a way to dump the fuel into the gas tank.
OK...one other thought/comment. I asked Holley if I could use the gas tank vent outlet for the return fuel...I would then use a vented gas cap. The answer: no, because the outlet is not big enough and you would need to somehow get a hose down to the tank bottom.
Let me know what you do!?