1st off you did good by getting another raditor....i think your problem lies in the speed your running water thru the block...
at this time is sounds way too fast.......this explains why it heats up when you start moving the car in gear.The manual water pump speeds up with the crank speed therefore starts circulating the water faster...The water is not in place long enough to draw heat away from the block..
what size pulley do you have on the water pump and what are you using where your thermostate goes ?.......Your gonna need a restrictor in place to slow the water down.....most people just take the center out of the thermostate and place it back under the water neck..
try this 1st before changing pulleys...the next step would be to change the pulley to a slower rpm pulley...but i think the restrictor will help a bunch
Brian
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Bjuice..
"I'M YOUR HUCKLEBERRY ! "