So what's the deal? Which carb is the right one?

cruiser said
there is a filled in hole that looks like a dimple between the two screw holes on the left side of the carb. On my Holley 1945, there is a metal nipple in this spot, with a rubber vacuum hose attached to the nipple which supplies vacuum to the heated air snorkel door. But since this nipple doesn't exist on the 902 carb, where am I supposed to plug in the vacuum supply for the air door?

That hose wants to see manifold vacuum. So you can tee it into the choke pull-off hose, or you can connect it to the vacuum tap on the rearmost intake manifold runner.

cruiser said
On the right side of the carb (second photo) the lowermost of the three holes is where the charcoal canister line plugs in. But this carb has no nipple there.

Just as well; do like this and you'll wind up better off—the stock '74 setup using the throttle plate as a halfassed purge valve was a cheap, nasty, halfassed way of doing it.

cruiser said
So would I somehow have to extract the nipple from my existing Holley 1945 carb and insert (screw? push?) this nipple into the new carb

Wouldn't work; unless the nipple was originally installed on this carb and has gone missing, the hole isn't drilled.