I recently bought a 55 Ford with and 89 Grand Marquis drive train with 5.0 EFI and an AOD trans. My question is: what fuel pump are you using and are you using a stock gas tank? More questions to follow. This is the first EFI car that I have ever had to work on.
I did an 5.0l efi swap in a 66 Fairlane and I used a Walbro external pump plumbed in right by the tank using the factory line. I then ran a return line and welded a fitting to the neck on the fuel tank and plumbed the return line in there. By doing it in the neck you can remove that part, clean and weld then it just bolts back in. Now in my 56 I believe the neck is welded to the tank already. You could cut it off an put it back on after with some rubber hose like the 60's cars had. Another place to look would be the sending unit to see of there is room there for a bung. There is also a drain plug on my tank so you could look at that for a return line although I like the other ideas better. Or don't monkey around and just use Aeromotives new setup. http://aeromotiveinc.com/products-page/new-products/18689-phantom-200-stealth-fuel-system/
I'm facing the exact same situation with my '54 Mercury Monterey, koolmercuryman. It's got a Lincoln 5.0 HO / AOD transmission combo from a Lincoln Mk VII coupe which is probably pretty much like your Mercury motor and EFI. I bought an aftermarket in-tank pump adapter (I don't have the details handy, sorry - but I'll get 'em.) from Summit (I think) that installs in the stock tank sending unit opening. It has connections for both the output and the return lines and hooks to any ignition-activated 12-volt source. I do not have it installed yet. I may cut a new opening in the tank for the fuel pump adapter instead of using the stock opening so that I can retain the original fuel level indicator sending unit. I've got the tank all dipped but have just not had the time to get this done. I'll do a post on it when it's all hooked up and running. (When I bought the car it had an external in-line pump hooked to a fitting in the fuel tank drain hole ! Yup. Hanging down in the breeze where any ol' thing could knock it off, causing a m***ive fuel spill and... !!!! The return line was hooked to the stock outgo fitting on the tank. It worked...after a fashion...but was a little too risky for me.)
The first couple years F 150's ran EFI, they had the high pressure pump on the side of the frame rail about under the drivers seat. Look for one of those. I would think for a return line you could drill a 1/4 inch hole in the fill pipe, run a bent tube in, secure it with a hose clamp and some silicone sealer.
ole don is right. even 94s f150 had frame mounted fuel pumps there relatively quiet good reliable pump.
I have the same setup in my car. I just teed the return right into the feed Line before a generic pump and filter. That way I kept the factory tank and didn't have to mess with it. Wasn't sure if it would work or not but 10k later and no issues. It's mounted to the frame rail in front of the rear wheel.
My current project is getting my fuel system/lines in. Most of the 4.6 crowd I consulted with went with an in tank unit where posible. The one that couldn't because of his wagon tank ended up having troubles on rapid acceleration and quick turns. He ended up putting in a cannister affair before the pump to keep the pump from ****ing air when the tank was less than 1/3 full. An in tank pump will eliminate that problem if it has a baffle, and will be quieter. My local buddies at a hot rod shop have had good luck with the Tanks Inc P series pumps. The actual pumps they use are Walbros. A minimun tank depth of 6" is required. Since most of our tanks mount against the trunk floors, they offer a recess box which can be welded in. That's what I went with. I'll just add a panel to my trunk floor for access should I need it. Prices are good also. I went with the pa-4, 235.ish they have the mentioned baffle included. www.tanksinc.com click on fuel pumps. check out their installation videos.
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