So I'm chasing an odd coolant leak in my B18/20 Frankenstein build. Block is an over-bored B18.

There is coolant slowly dripping from the bell-housing under the car. As the car sat all winter, the coolant drained enough to get below the level of the leak, so it "stopped".

At first I thought it was the coolant pipe and the 90 degree fitting on the rear of the head. Both were somewhat corroded and testing it showed that they had small leaks when the hose clamps were tightened down. Put in a brand new coolant pipe and a brand new 90 degree pipe...no leaks from there any more.

Filled up the system again, back of the head and top of the block and bell-housing are bone dry, but the coolant drip under the car started again.

Question: Is there a coolant plug of some kind at the rear of these blocks? The only freeze plug I remember seeing was the one over the back end of the cam. But I think I remember seeing a small npt plug back there, and am wondering if that's leaking. If not, then is it possible I have a crack in the block that is allowing the coolant to drip out by the flywheel (which would utterly suck)?

I plan on pulling the trans and flywheel once the weather gets consistently better, but I'm a bit vexed as to what could be causing this.
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Tony - 1958 Volvo PV444, 2012 Volvo XC70 T6 Polestar