when you fitted the thermostat are you sure it is bottomed out into the block ? they are difficult to seat without quite a bit of twisting right to left and at the same time giving downward pressure. i used red rubber grease to help with this. the other thing that springs to mind is have you nipped an oring on the edges of the housing of the stat recesses for the rings that are quite sharp and need a needle file on them to take the sharpness off. you need to find the origin of the leak first before you can move forward with the misfire. this is your first job. with a v full of coolant it should come to light pretty quickly as to the cause. sometimes the inlet manifold can be warped if not torqued up in the correct sequence but i doubt that. you must find the leak....
