Leaking coolant

Yes. The 318 c.I. Has two core plugs, (aka freeze plugs), at the back of the block. The one on the driver's side leaks most often and the right one looks like new, but replace both as long as you have to go through that much work. Casting sand that never gets fully cleaned out on the assembly line seems to collect by the left core plug, and I assume that is part of what causes this problem. Another contributing factor is insufficient cooling system maintenance, something I'm guilty of. The additives in antifreeze wear out in about two years, which is why we're supposed to replace it every two years. Those additives include water pump lubricant and corrosion inhibitors. It's the lack of that corrosion inhibitor additive that leads to the corroded core plugs.

Personally, I would not replace the core plugs on the sides of the block until that becomes necessary. The exception would be if many of them look rusty. Normally it is common to find one or two corroded through while all the rest are nice and solid.