I was finalizing the 350 Chevy rebuild and ran into pan gaskets not fitting.

I have both left and right dipstick tubes. I discovered that my 87 tube was not going to work in my block (passenger side) and will have to purchase a new tube and dip stick to be located on the drivers side. With this gasket set, one is going to be right side Blue up (FelPRO) and left side Blue down. I thought this was strange but this is the only way they fit. Any explanations on this?

Also what is the best plug to put in the unused dipstick hole?

Thanks!