My "opinion" FWIW
I'm sure all of you at some point in time had an opportunity to put a length of garden hose in your old man's fuel tank and suck on the outboard end to syphon out some gas into the handy dandy fuel tank and if you were lucky, you managed to get the fuel to flow. if you were unlucky, and a air bubble got stuck at the upper part of the hose then no fuel was flowing. Something similar will happen here. When you have a fuel line that's running somewhat level in certain parts what you've created is something like a babbling brook. The fuel will run downhill and then will burble across that fuel filter until it gets to the other end of the fuel filter and then flow down towards the carbs. The more full the fuel tank is, the greater the "head pressure" of the fuel on the fuel line. A full tank will have just over 12 inches of head and 1 atmosphere of water pressure (14.7 lbs) 34 ft So 1 foot of water pressure would be 1/34th of 14.7 lbs = .43 lbs (less than 1/2 pound) and gasoline is 25% lighter than water so reduce that pressure down to .324 lbs/in sq.
So, on a full tank .324lb/sq in got the fuel down to the filter and then the air bubble (hiccup) Now the fuel will flow across the filter and then flow down to the carb and there's next to no head pressure here because the fuel filter is a couple of inches above the fitting going into the carbs.
If you wanted to do a little test for shits and giggles-
Pull the fuel line off of the carb fitting and allow the fuel to flow for 30 seconds with your existing set up with the filter on the horizonal and a bubble of air in the viewing of the filter.
Next, pull the fuel line off and block the end and lower the fuel line down and allow the air bubble in the fuel filter to rise up and enter the tank basically filling the fuel line with fuel. then raise the fuel line up to the point where it enters the carb fitting and then allow the fuel to flow out into a container and let it run again for 30 seconds. My money will be on a significantly greater flow of fuel when there's no air entrapped in the fuel line.
so the goal would be to route the fuel line in such a way that under no conditions can air get trapped in the line or if by chance air manages to get in, that the air will flow uphill and purge itself in the tank.
Somewhat long winded, but if you read my posts I tend to go deep into the engineering/ science of things. It's my fascination.