1971 Demon with some coolant issues

I picked up my first Mopar project yesterday... a 1971 Demon, 318, 55000 original miles, one owner (literally a grandma who only drove it to church on sunday..). It has been sitting covered outside for approximatly 10 years.

The previous owner would start it up once every 3 months or so, let it warm up, then shut it off. He would also change the oil periodically. Last time the oil was changed there was water in it. I towed it over to my garage, pulled to plugs and ran a compression test:

1: 130psi
2: 135psi
3: 130psi
4: 130psi
5: 100psi
6: 130psi
7: 100psi
8: 90psi

As you can see the back cylinders look like they have a problem. It has been suggested to me that I change the oil, refill the coolant, start it up and watch it carefully. Does anyone else have an idea of where to go from here?

I am really hoping it is not a cracked block or a serious problem. Because the 318 only has 55,000 miles I am hoping that there is still quite a bit of life left in it.