Hi Tim,
I'm running a 1/2" V-belt in the 3/8" pump pulley and in a 2 groove modified 250 balancer. Have found that running a 7 pound cap and not overfilling the radiator works better than topping it off and trying to run a 13 pound cap.
I added a pusher fan for the a/c unit in slow heavy traffic conditions, but haven't found I needed it. I would imagine you could probably go up to a 17 or 18" fan with that same blade mounting configuration, if you felt the need, especially on a larger vehicle like his firetruck.
Dave
