AmericaWestLines, welcome to the forum!

You are correct that adapter pins would be needed. Rather than trying to adapt the 2 types of track for a short section, it would be easier to just cut an '0' gauge track to size.

A minor issue here needs to be addressed concerning the identification of the various types of track Lionel made. The smaller track is known as '027' track, which unfortunately identifies both the minimum circle diameter of 27 inches and the type of track.
The larger track is '0' gauge track, which has a minimum diameter of 31 inches.
Both types of track come in larger radii. Technically, there is no such thing as '031' straight track, it would be '0' gauge straight track. You would use '031' curved track on your layout. Unfortunately again, we would also refer to the smaller straight track as an '027' straight section, and you would use an '027' curved track on your layout. Confusing, but that is how the track was marketed.

Now back to the trains!

Larry