Steve,
typically when a reground cam is used with the short lifters like this, the bosses are machined down shorter to allow for the lifter to be exposed when up on the lobe.
I know that doesn't help right now.
So, to cheat fate with the head on, find a very large "C" style spring compressor that can reach over the head and catch the spring retainer. Be careful to not bent valve against the head too hard, it should allow you to lift the lifter up and catch the flats.
Enjoy, John
PS. done this before once, lernt my lesson