I understand your concerns about the oil weight but the bearing clearances not displacement determine which oil is best. Tight clearances need lighter oils, and modern oils are very good, much better than when the engine was designed. Heavy oils cost starting problems and fuel economy unnecessarily.
I have installed a lot of block heaters but I do not have an opinion on which is better. Most have been sourced from NAPA with unknown manufacturers, most have been completely reliable. The factory installed ones have been in exactly the same reliability class as the aftermarket in my experience. No OEM makes their own, they are all purchased out, so I see no reason to expect any difference. The real problems come from owner malfunctions, damaged cords, failure to maintain the coolant in good shape, installation errors, things like that. The OEM in my current truck is 20 years old and still works fine.
There really seems to be no reason to consider installing two, unless in an extreme case the extra heat is needed. One 600 - 650 watt unit should be plenty down to about -20F. The failure rate is so low that the problems associated with installing a second heater would keep me from doing it. The reasons for failure of the first unit generally apply to the second so if one failed, the back up would likely as not be failed too or soon after.
Also, many engines only have one location suitable for a heater, I am not sure if your engine is that way. Often there is an exhaust feature or the starter or other item blocking the outside, or a cylinder wall blocking the inside of a certain apparently convenient core hole. The one that is easiest to get at and would be ideal from an installation point of view is usually blocked internally, no room for the heating element. If the exhaust is close, I would suggest a heat shield to protect the heater parts. You will not be able to pick and choose the location you would like to use.
It might actually be getting difficult to find a heater for an engine as old as yours, I would expect that unless the same unit fits newer models, no one has made one for many years. It is also possible that heaters that would fit are not listed as such anymore, the catalogues have been changed to drop out the old applications whether or not the unit fits. You might have to go with trial and error for fit, or try to find an old book. The heater manufacturers might be helpful with fitment suggestions.
On the stand installation is a very good idea, some installations in the vehicle are quite difficult. Silicone in the hole and the o-ring areas of the heater will preclude leaks. The hole should not be painted as that will likely prevent the heater from entering, they generally fit very closely.
When all other efforts fail, a lower hose heater, a glue on heater or an external "tank" heater could settle the issue. There are magnetic heaters as well but they are of low wattage to protect the magnet and not really suitable for low temperatures. All of these have their own problems, the frost plug heater is the way to go if you can arrange it.
A properly installed heater usually would not need to be run over night. I put a timer on the outlet I use and set it for 3 hours before I plan on starting. The 600 watt heater in my V8 warms things up enough in that time to make starting a non issue and it saves a lot on the cost of running the heater.
