[Sirrotica]

I missed the link to the overflow hose video on the first read.

Having seen this symptom many, many, times over 50 years of working on cars and engines as my profession, you have a head, or head gasket leaking. A distant possibility, would be a cracked cylinder in the block. No need to look further, pull the plugs and you'll find which cylinder, as the plug will be steamed cleaned and have zero deposits on it.

I worked in a Buick dealership for 3 years and ran in to at least a couple dozen buicks with blown head gaskets. Common on 455 and 350 buicks, and the short lived 4.1 V6, basically a overbored 3.8.

I couldn't tell from the video if you had cast iron heads, but the later cast heads had a propensity to crack between the valves. SBC cracked in the same area.

Finding no evidence of coolant in the oil doesn't really pinpoint anything, I've seen buicks with blown head gaskets so bad you could put a garden hose in the radiator and couldn't keep water in them, yet show little, if any trace of water in the oil.