For the valve cover, I use black RTV on the head and nothing against the valve cover. The gasket glued to the head with RTV creates a dam which allows the oil to go down the drain holes in the block instead of under the gasket. Since the head in a very smooth surface, RTV is very easy to remove from the block later on. The valve cover will need to be checked for a gapless fit on the head before the gasket goes on. That is where you'll need to develop tinsmithing skills.
The side cover will also need some tinsmith work.
Good modern cork is a good choice for side covers and valve covers. It will squish to conform to the very mild imperfections in your tinsmithing efforts.
Super 300 is a good call for keeping 90 wt oil inside of the rear cover.
