I am not an expert on these engines but I have had extended cold weather experience in Minnesota, about 50 years of driving. SAE 40 will lead to difficult starting without preheating, and could lead to loss of oil pressure at the far ends of the oiling system. Extended warm up times can cause drain back problems with attendant pump problems. Preheating will help with starting and oiling all systems in the engine. I can see no reason for using heavy oil in a "new" motor. I would say you have the breakin lube situation covered and suggest that for at least the first couple of oil changes that you check the oil filter for metal.
I would stick with the builders recommendations on oil type and weight. Main bearing clearances are the most important factor in oil weight, with cold starts second in my view. I run 20W50 in my 1967 400 cid SBC but it is not run in the winter. It does have large main clearances for more flow for more bearing cooling, on the recommendation of my engine builder.
The block sealer sounds like a good idea any time you change the coolant, you may need to flush your heater core as it tends to filter the coolant and clog. I would not worry about the valve seats, additives for the fuel are available if you want to spend the money for them. In some areas of the country they are not legal for road use.
My thoughts are not definitive, only opinions.
