The 1955 235 engine I bought had lots of rust in water jacket. So far I've blown it out as best as possible with air pressure, but plan to use a solution of muriatic acid I've seen on here and then flush with a soda solution. Right now the problem is that I can't get air through the port where the block drain screws in. I've even put an air fitting there and pressurized with 120 PSI with no air coming through. Does the water passage go toward the rear of the block? I've tried to use some stiff wire to scrape out the build up. What ideas does anyone have about getting flow through this passage?