chrysler 200

new member here i have what i think is a late 60s mustang that was given to me
by a friend that knows i like getting old stuff running. it has a chrysler 200 that is 273 cubic
inches. the last registration date is 2002. i pulled the heads and have ordered the parts to do a full valve job. i noticed some dirt in the water passages on the block so i began to
clean it out with a piece of wire and a shop vac. as i poked the wire in the rear water jacket
on the port side the crud started to fall out of a 5\8 hole on the oil pan surface of the
block straight down in the bottom of the boat. this hole is machined and partially covered by the oil pan flange i know i will have to pull the motor to fix it but i cant find any info
on what goes in the hole the freeze plug kits have two small plugs but they go in the rear of
the block facing aft any ideas would be great. philip