Did you happen to put new pcv gromets in your valve covers? Were the gromets a complete open hole or did they look like a plug. When I did my 400 I put new gromets in new valve covers and the bottom of the gromets had to be cut open for ventilation. If you can not suck out the pressure through the pvc it WILL build up enough pressure to blow oil out the gaskets and excessive pressure will build up in the radiator. I know this from previous experience.