There are several methods used to block the round POE vent holes, I've seen some building use screen/epoxy, just plain epoxy, etc. I like to use screw in set screws, and lock them in place with a drop of loctite for insurance.
The wells themselves need to vented internally back to the main fuel bowl.
The only vent required in the airhorn is the round tube, all others can be blocked off.
Keepig fuel from coming up around the secondary metering rods is simple, use a good quality black electrical tape of the holes during assembly, they push the rods thru them. Lubricate them with grease so they don't stick.
I've seen foam used there as well, with the rods poked down thru it.
Float level will depend on the size of the fuel inlet seat, float size, and fuel pressure based on engine requirements. Fuel pressure will have an effect on the level, all else being equal.
A good starting point is apprx 9/32", small float, .135" inlet seat, and 5-6 psi fuel pressure.....Cliff