If this didn't help (additional cooling capacity) then one possibility is the problem relates to coolant flow.
So I would focus on the following.
Is the pump and thermostat working at all ?
Test: From cold, remove radiator cap and start engine.
a. The pump should pull the fluid level down.
b. When the thermostat opens coolant will flow into the top tank.
Is the pump impeller matched to the pulley ratios? In other words is the pump turning too slow ? A/C pumps got a different pulley ratio.