Best way to seal intake and what gasket?

All this magic goop wont help much unless you have proper preparation.

I use denatured alcohol on clean microfiber cloths and lots of both. What I would do is pull intake, clean the whole gasket areas on everything with lint free cloth and lots of denatured alcohol.
If you have debris or any silicone or gasket material, this will need to be scraped off. I would do this slowly with someone holding a shop vac nozzle right next to your area you are scraping to suck out all debris as it is chipped off the gasket surface.

Then I would duct tape off the camshaft valley inside below the gasket line on the rails and over the intake ports. Then I would get grey scotchbrite pads soaked with alcohol and scrub the block intake rail front and back until it is shiny bare metal. You will want to use the shop vac to suck out any particles while scrubbing with the scotchbrite.

Wash again with alcohol on microfiber cloths, let dry completely, just attach the gaskets to the block with anything that others recommend as the best sealer. I have been using black permatex.

The cleanliness and a roughed up surface is key to any gasket sealer maintaining adherence to both the intake and block along that rail.

Just like any process involving adhesion, prep is 80% of the work.