Rosco,
The seals on the water pump will prevent it from free wheeling. This is normal. Are you using an OEM pump or one of the high volume aftermarket pumps?
What kind of pulleys are you using? If March, the street rod ratio works better on street motors than the Hi Po ratio.
What kind of fan are you using? Although, at speeds over 30 MPH, your fan shouldn't come on at all. If fan isn't doing its job, motor will run hot, or overheat at low speed.
What kind of lower water crossover are you using? Any chance of it collapsing and restricting flow? Long lengths of lower hose need to have those funny looking springs inside to prevent collapsing. Exhaust tubing makes good lower water crossover. (Pump inlet on drivers side, radiator lower outlet on passenger side)
Air, or exhaust in the coolant. Any bubbles in coolant when you hold upper hose up with engine running and garden hose keeping radiator full? Coolant in
oil? Water condensation on dipstick? breather? PCV?