Hi, here is a write-up I did on my 383 cooling issues. You mentioned you are using the 6-vane pump, you might want to get the 8 vane version.

The 6-van is used with an A/C car with a smaller pully, which spins the pump harder. The 8-vane pump is used with a slightly larger pulley on a non-A/C car. Which pulleys are you running?

You problem to me sounds either coolant flow related or more likely airflow related (especially considering your clutch fan is not doing the job).

This thread is now "archive status", which means no pics, which sucks... if you want the pics of the pump I can dig them up.

http://www.bigblockdart.com/forum/archive/index.php/t-20229.html