The valve guides are supposed to be installed with the head heated to 300 degrees while the guides are frozen.

The problem isn't in the head, it is in the bottom end. Your piston rings didn't seal for some reason (shitty honing job, incorrect gap, or improper break-in). 10% leakdown is hardly acceptable on a freshly built motor.

It sounds to me like the rings were washed out due to the excess fuel upon startup and never had a chance to properly seal. Regardless, the engine is going to have to be torn apart and rehoned and rerung to run properly.