You seem to be trying to put a wire feeder on a constant current power supply. Generally wire feeders are used with constant voltage power supplies. There are wire feeders that can be used with constant current supplies (I downloaded the operators manual for lf-72 http://www.mylincolnelectric.com/Catalog/operatorsmanualdatasheet.asp?p=33858
and this does not appear to be true for it). Make sure the salesman you work with is aware of what you are trying to do and ask for a written warrenty that the equipment will do it
Bill