The plunger should not be an issue if it extends under pressure. You and your dad or a friend can plug up all of the outlets with your finger tips and use some compressed air at the inlet.

I build a number of heads and the "old" stem seals are always "free" but still work. The new stem seals always hold tight for the first few hours. I use Viton on the exhaust side. I suspect you are fine if you can pull the plunger or make it move with 50 psi.

That said...you MAY need to make an adjustment to the CO/idle screw as the O-ring frees up.