[Brian]

A peculiarity of 35-36 engines is that at high flow rates, the cylinder head mounted pumps tend to push coolant up and outta the overflow. Fitting thermostats in the top hoses can slow this down considerably. Even then, coolant will be pushed outta the overflow as it expands with heat, until it finds its own level. In my 35 I run the overflow into a catchcan; this ensures the radiator is always filled to the brim.
I think the surface area of a Model A radiator would be somewhat smaller than a 35-36 rad, although you've had the tanks modified, does the core have enough capacity to cool your larger engine?