All 318s and 5.2s can interchange flex plates if you are going carbed, they are all internally balanced.

360 LAs had the weight on the converter. Early 5.9s suposedly had the weight on the converter and it was smaller than the 360 LA weight, 120 grams for the 360 and 91 grams for the magnum. By at least 94 the 360 had the weight moved to the flex plate and the converter was neautrally balanced. The easiest way to put a 5.9 where a 318 came from is to use a 94+ 5.9 flex plate and then you can use your converter from the 318 without any other mods.