The intent of the filler neck elevation is to make it the highest point in the
cooling system. This allows the complete filling of the radiator, and the
elimination of air pockets in the system with flow driving them to the cooling
system's highest spot. Hopefully it is one that lets it out, like the filler.
Then topping it off removes the rest of the entrapped air.
The Tiger surge tanks is never full, without a recovery system, but the water
level inside the tank is supposed to be the highest point. I am not positive it
is, but in the Ford and Corvette tanks it is.
Some have claimed that the filler neck really ought to be on the other side, and
I've seen aluminum radiators just like this. In any event, they have added a
filler on a right angle tube at the top side of the right tank, yet below the
line (with engine rocking).
Most Speed Parts merchants carry a filler neck on a hose beaded "Tee" with a
central cap. I am not sure this is high enough, and could result in the water
level inside your cross flow not running through the upper tubes, although mass
flow rate might just do it anyway.
There are "half size" recovery bottle systems now that handily fit on the
left fender, even with a stock tank, and under the front fender well, as well.
Much harder to fill or observe under the fender, but you could but one on both
sides and "T" them on outlet. This would allow a doubling of fluid contained,
only the one inside the hood need be filled as the water crosses over at the "T"
junction. Try to keep them at nearly the same height.