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Y is just the coding, IY blocks were not used in the '70s.
A 360 is a FE series, what I would expect in a '74
FE intakes also go a ways under the valve covers.
How many bolts in each valve cover?
As long as the bypass is open, unrestricted, some coolant will bypass the radiator. Once the T-stat opens, shuts off the bypass, coolant can only flow through the radiator.
I appreciate the input but this is a 351m and you can see from the picture and casting number that is a fact. I know how the crucial the correct thermostat is in the proper operation of the cooling system for 335s otherwise I wouldn't have gone the extra mile to order the correct one from t-meyer. I have never wanted to be wrong more in my life because something as simple as a restrictor plate would be an easy fix for me.
Update: March Performance 1618-08 water pump pulley install
I received the March underdrive pulley for the water pump and got it installed. It took a bit longer than expected because I had to fab up an alternator bracket to clear the new and fatter water pump pulley. I attached some photos on the differences in the pulleys in case anyone is interested in trying this pulley themselves. The truck has been idling for some time now and the temp is holding steady. So far so good.
