Alex, the idea is to provide a route for all the air in the system to return to the expansion tank (highest point in the system) from all the other high points in the system such as the back of the heads and the top of the radiator(s). Only one route is necessary so there's no reason not to "T" the distant ones like at the top of the radiator. I used AN4 for these and I think bigger would not be better in this case. Smaller will work but I have iron blocks and they tend to produce crud over time that might obstruct a small port opening. AN3 would be fine also.
I have done my GT40 this way and never had a air in the system issue since. Just fill it up with coolent and it will self bleed in a few minues. I also did my SBC in the SLC the same way. 2 AN4 lines, one from each side of the radiator T'ed together and then a 1/4 inch alum tube run back to the expansion tank. Also the engine has three vent points. Two at the rear of the intake manifold on each head, and the third at the front of the intake manifold using a spare coolent sensor port. All three are "T'ed" there and a second 1/4 inch AN line is run to the expansion tank.
The expansion tank is located as high as possible in the engine room and the cap is within a inch of the highest point on the car just under the rear body work. The radiator cap vent line goes (port right at the cap bung) from there to a over flow catch can located lower in the engine room and it can be drained from under the car from a pet cock.
I used a AN8 line from the return (inlet from radiator) side of the water pump to the bottom of the expansion tank to vent the system.
The only other thing to think about would be a heater core if you have one, I don't. It it doesn't bleed itself when the lines to it are fully open, I don't think so but maybe, then a bleed line might be used at the higest point in the heater system also.
Check out my SLC thread for clear pictures.