I did the rubber replacement on a 307 Olds for my brother, it was good fore the next 3 years he owned the car. If you live in a cold weather country they make a block heater
that will look just like the rubber freeze plug.
+1 To this idea. Plus the block heater plug does not require removing the intake to install. I absolutely love the core plug block heater in my 225 valiant. Easier winter starting and much much faster warmup/heat. I recall the slant using 1 5/8" freeze plugs. here is a link to a suitable plug style block heater