The float level, when set correctly, provides fuel in the main jet very near the top of the jet, just below the bridge in the carb throat. If the float fails open, the fuel level in the carb bowl will be higher than the top of the bridge, and fuel will flow out of the jet and down into the engine. If your fuel tank is low, it may only drain the bowl, but if the fuel level in the tank is higher than the carb, it will continue to flow by gravity until the level drops.  I'm not absolutely sure about a HS2, but HS4 and HIF carbs have their overflow ports well above the bridge level, so you won't see fuel coming out them. The only time you'd see fuel coming out the vent/overflow is when the fuel pump is operating.

Working on mine, before seeking expert advice here, I purchased the correct mini fuel line and also found it unusable for my set up. I have 3 lines on the left float lid and 2 on the right. they are feed to the left (in) out to the right, and over flow the other is feed to the right and overflow. I invented a kind of W or M shaped stainless crossover line for proper stub hose connections, and to clear the Carb linkage, that also bolts to the heat deflector.

A separate question, will the over flow tubes prevent the crank case filling over the winter with full tank storage in the case of a leaking seat, or stuck, sinking float?

Your front carb should have something like an AUE 268 or AUE 269  float lid which has the correct in and out line connections. The one you have is a single line set up with an overflow fitting (not an output line).
You could use a much smaller filter than the one you have or re locate it under the boot floor as Steve said.

You could switch things around and feed the rear carb first that would allow you to fit the fuel filter in the rear of the carbs, might look a bit messy though.

As Doug said you need the brass tube to connect the fuel line it also has two clamps to attach it to the upper carb mounting bolts.

5/16" coarse sounds correct for the bolts.

Edit: I just looked at two sets of 1.25" S carbs on manifolds and both of them had single line feeds. These you would use with the correct copper line.
I tried to look on the SU site to give you a number but it won't load at the moment but the front lid should have the T shape connector with the feed exiting at the bottom of the T between two of the screw holes used to mount it if that makes any sense. This will give you the correct orientation for the fuel line and copper line.

The factory dual carb setup would have used a T-pipe instead of a T-fitting.  See our host's part #12G292 for the factory pipe. 

I could not find a good picture of that pipe installed but take a look at the image linked below.



You will see the pipe run along the bottom of the picture.  It turns up and to the left just off the image.  Note how the front bowl lid is oriented so the bowl lid brass pipe is pointing down and to the right.  Short bits of rubber fuel hose join the T-pipe to the bowl lids.  As Steve said, it's difficult to connect an engine bay mounted fuel filter to the T-pipe.

The second brass tube on the front float bowl lid is probably an overflow vent.  Confirm this is the case and use a length of fuel hose to direct any overflow back to the firewall then downward so no fuel can splash on the exhaust.

Thank you for filling out your profile. Don't know who rebuilt your carbs. It would be nice to get your parts back. I would suggest you locate that fuel filter at the rear under the boot. There is more room to work with back there. The normal hook up between carbs would be a tube forming a Tee if you like with a long part going to your right carb a short tube going to your left carb and the long tube going on toward the area where you have your filter. That should be connect to the supply line via a short length of fuel line. I'm going to go out on a limb and suggest the extra tube coming from the lid that was not yours is a bowl over flow off a MGB which would get a line to direct over in case of a stuck open float valve away from hot exhaust manifold. I must have a hundred float bowl lids. I'd say the bolts that hold the air horns on are 5/16 course. Hope that helps. Steve (CTR)

I just got a new intake manifold and had my carbs rebuilt as part of undoing some of the kluges from a previous owner. I need help with a few different things as I’m trying to get everything back in working order.

1.  
2. 1. One of the carbs came back from the rebuild with a different float lid. I used to have one connection per float and the fuel line ran from the filter into a T and then to each carb. I do want to get rid of the T in the fuel hoses, but I feel like the float lid that they installed must be for a RHD car. No matter which orientation I rotate it, it leaves fuel lines running at impossible angles. I have been looking at all the options on the SU site here, but I'm not sure what the right solution is. What float lids would be correct for dual HS2 carbs on a LHD car?
3. 
   2. The other issue I’m struggling with is I can’t figure out how I can possibly connect the output of the fuel filter to the float lid nearest it. You can see the filter on the right of the attached image. What am I doing wrong? I can’t see any way that I can make the connection without some crazy bends in the hose. Should the lid with the two connectors go on the far carb instead? Is this why they used a T? Is there a better / more compact filter I should be using?
4. 
   3. Finally, the bolts for mounting my air filters got lost somewhere along the way and didn't come back with the carbs. What is the proper size for the bolts that connect to the flange on the carbs?

Thanks!
Scott