Your easiest way to use a 727 converter would be to find a 2 wire 518 . 2 wires are non lockup , look for trucks or vans '89-'91 with a 2 wire connector . The 518s that require a lockup converter have a 3 wire connector . Any 2 wire 518 will use a 727 converter . Your question was can a lockup 518 be converted to a non lockup . I think the answer is yes but would be more involved , it would be easier to look for a 2 wire transmission . I have a 2 wire 518 in a '38 Plymouth with a 408 engine . I am running a PTC 727 converter .