340 rear main seal question

They call the little extensions on the edges of the seal "wings" and the little white plastic tab with it is for ensuring the outside seal edge doesn't get trimmed off when sliding it into position in the block. (A shoe horn if you will)
You can't offset this seal, and the tabbed (winged) half goes in the main cap.

(MAKE SURE you put it in the correct way or it will leak like a pig)

A tiny little dab of silicone on the very end where the seal halves meet and a TINY bead of RTV from those wing tips across to the pan seal area is used.
TINY as in a film where the color of the RTV is barely visible is fine, and you want to get all the way across and around the corner the seats into the block.