Don't think I'd trust it with fuel.
Easy way to plug the fuel line is to stick a piece of fuel hose on it, shove a 5/16 (or whatever size your line is) bolt in the end and hose clamp it.
You said you found TDC on the compression stroke (with a piston stop, I assume) of #1, but where was the timing mark on the balancer at that point?
At TDC, mark the balancer at the "0" mark on your balancer. That is your new timing mark.
Still no power? Are you 100% sure your distributor isn't...
Which is a freakin' shame- everything now is just a homogenous all-looks-the-same blob of a vehicle. If they took the signs down you wouldn't even be able to tell what brand dealership you're at.
I'm gonna take a WAG and say it's for a bulkhead plate or grommet for a pass-through on an aftermarket air conditioning system.
This one's for a 1 1/2" hole- is that about right?
As said previously- whatever it is, it ain't factory. At least for anything I've ever seen.