A cool running engine just doesn't happen. It looks like you've done a good job on your rebuild but I see a couple things that would contribute to your low speed cooling issues. Several have been touched above. IMO....using the "short-pump" and the "pusher-fan" set off alarm bells. I tried the "short-pump" route and soon learned that you needed the mechanical fan high on the radiator to move air where the water enters at it's highest temperature. The adaptor plate and correct "early" pump gave me clearance to run a mechanical fan and combined with a 6 blade fan (Summit FLX-1615P 15") moved the fan up higher on the radiator, gave me the proper waterpump rotation speed, and my cooling issues were history. Secondly, a pusher fan is not all that efficient, especially if mounted lower in the radiator. You're pushing air through about 1/3 of your radiators surface area.
So what would I do to correct the problem? Adaptor plate, early 216-235 pump with 5/8" belt(NAPA), 15"-6 blade fixed blade (no flex), 180 thermostat (with 1/8" hole drilled in outer area), 6-7 pound cap, burp the cooling system, use a 25/75 coolant mix with distilled water and fill the radiator only until you cover up the core.
With this setup if you are still having cooling issues it's not because your cooling system is mismatched.
Dave
