Too much air pressure on valve cover breathers

Not a surprise at all for a stroker engine to have a fair amount of blow by. You have just increased the amount of volume of air that is being pushed around in the crankcase and unless you give it somewhere to go it is going to build up pressure. A big oil pan can help but the best bet is to go with a remote breather setup. Run a couple of lines from the valve covers over to a puke tank with a breather on it. You can also run a remote PCV valve if you want. Moroso builds remote tanks as do a bunch of other guys. Very common problem that has been around for years.