have just put my 351 into my xf . Motor has been recond a while ago then was left sitting for a while in doors. i fully did all the gaskets befor i put the motor in. Headgaskets , sump ect. New 3 inch alloy readiator and water pump. new hoses ect.
Now the problem is it seems to be building pressure in the cooling system. every time i take the rad cap off there is a release of pressure.
Any ideas????

How much pressure? Is the engine dead cold when this happens? Is the cap new? what pressure is it? what thermostat is in it?

Were the head gaskets fitted correctly???....was cooling system bled properly???....is there a 2.5mm hole drilled in the t'stat??? How is the impeller to timing cover place clearance??? How is the impeller to pump housing clearance