Hope you can figure this out along with the rms or pan gasket leak.
Regarding the valve cover grommets, I use the same ones you have on my 78 400 engine, the slot in them is not razor thin, its the full width of the grommet & is almost 1/8" wide, maybe a little less, but you can just about breathe freely through them so they probably aren't causing the leak issue or excessive crankcase pressure. By all means try open grommets to see if they help, but Ive never had an issue with them causing a rear main or other oil leaks.
https://www.moroso.com/breather-grom...-package68775/
They do make another type of baffled grommet that has a much bigger opening with a piece of foam in it to stop oil mist, if you also have mist out the breathers these might help a little & provide better breathing than the ones with a slot. On this same page you can also see the k&n type breathers that are fully open with the pleated material. I also use the cheaper chrome breathers that work fine, if they get loaded with oil mist I just rotate them 180°... never had an oil drops from them though.
https://www.ebay.com/itm/404113941017