The Compleat Fantasist - How To Convert The Major Fantasy Role Playing Systems To Each Other (1980)

 From the back cover:

AT LAST!...no longer need characters from one system to be stuck there. And never more will you have to arbitrarily decide how to change personal statistics to fit a different set of rules. Fantasy roleplaying veteran Mark Wagner gives guidelines to help convert many parts of each system the others, and describe aspects that don't concert well.