actually the wiper motor said to be for 58/9 that previously had vacuum doesn't work on the 55-57 trucks with vacuum ... don't ask me why, cuz with a number of 5-7 trucks and a 58 I can't find any difference, tho my 58 may have had OEM electric
you don't need a "conversion kit" for 55-57, just need the elctric wiper motor all the vendors should have for your years - it's shown 3 places in the LMC catalog, one of them looks wrong, the other 2 the same, may be the right one - American Classic and Jim Carter have them too
Bill