I am in the process of replacing the 3 1/4" baseboards in my 60s era
condo with plaster walls. When I removed the old boards in lieu of 2X4
studs there is a 2X2 runner along the base of the wall that the boards
were nailed into.

My question...I want to install 5" baseboards for a nicer look. Is it
better to nail the new baseboards into the 2X2 at the bottom of the
board and use an adhesive such as liquid nails to secure the upper
portion of the new boards or would it be suffice to just nail them in
and use caulk to cover any gap? To nail the upper portion of the
baseboard I would be nailing into a plaster wall wich I doubt would
hold very long.

Or...is that just too daunting of a task and I should just put 3 1/4
back in?!!