Rather than adding ZDDP, I think you're better off using an engine oil that has enough ZDDP already in it. Excessive amounts of any additive can lead to engine deposits. The Corvair Flat Tappet Engine Oil article recommends Heavy Duty Engine Oils (HDEOs) with a CI-4/SL ratings (rather than CJ-4/SM) ratings and these are commonly available.

I've written an Engine Oil article for the ACCCC website and I'm recommending 0W-30 HDEOs for our club member cars requiring 30-weight oil.