56 312 Coolant residue on rear main cap.

[My Old Blue]

Was able to pull the engine oil pan from my 56 Country Sedan's 312 motor to get a first look inside since probably the early 70's when the motor was replaced. Right off the bat I see coolant has pooled on my rear main cap. As you can imagine not what I was expecting to see or even seen come up in my forum searches.

Oil is clean and when I did change it coming out of 30 years of storage was not milky then nor now. This a new leak and one I had notice coming from back of block when I was pulling transmission and no real indication coming from a freeze plug or head gasket.

Is it possible the evident leaking heater valve on top is leaking coolant to back of block and somehow pooling and through a seal or possibly past the distributor and running down the oil pump shaft depositing on real main seal area?

I have included a few pictures from today's reveal and hope I can gain insight where to start looking or suggestions.

Thank you