plugged 955l radiator core clean out recomendation

working on a 955l 85j series radiator has been neglected for years, the out side is the worst its like asphalt in the core, is there a way too soak it in a tank, was thinking 50 gal barrel, with some sodium carbonate" arm and hammer" and a little heat to try and loosen it up its like plugged 40%, but some air is coming thru , need to get this small job done, can get a core from general rad for about $ 1,000 not bad considering cat is 2,400 thanks everyone