Never ran into a block, left or right side, that wouldn't fit a rubber expansion plug all the way, odd
Yes, the rubber expansion plugs work great, only reason car makers don't use them is that they are 10 times the price, lol, $1 vs $10
Get the old one out, you CAN push it inside the block it WILL NOT hurt anything being in there, it will just sit at the bottom of the block it can't block any passages
As said clean the hole
Put in correct size expansion plug and tighten it up
Do not over tighten it, its only holding 16psi at most