?Block Flush info?

What is the best stuff to use/& way to clean out the water p***ages in a block, that has been sitting for about 20 years with some water in it. It is an early GMC 292 and since it is in a 1937 Wheeler there is no radiator. Can some type of "secret sauce" be poured into the block, let it sit and then flush it out? Any info would be appreciated, as I am trying to get the motor running for the 83 year old guy that owns it.....yup this might be a little OT, but it is an early GMC block. Thanx for any ideas.

 

water and muratic acid.then flush with water thoughly.