What's the best way to find a dead cylinder?

Vac gauge to pcv hose? Why don't you just disconnect the vac line that goes from base of carb to the air cleaner housing?
That's straight manifold vac and same size as most vac gauge nipples. The pcv hose is twice as big, without the right adapter you'll leak more vacuum than the gauge need to do it's job

And unless you have a museum piece I wouldn't have worried about the peanut head being "incorrect". The peanut head has 6 less potential oil leaks.