Hi,
First, besides eliminating other possibilities, how would a leak down test tell you anything about the valve guides?
Though they should help, the stem seals are a temporary fix for a 300E with that kind of mileage. The problem is that as the guides wear the hardened(less pliable)stem seals are bot able to deal with the added axial movement. You might get 10K miles with less oil consumption, but plan on a valve job...nothing lasts forever.
Regards,
Randy D.