I would check the junkyards for a filler neck setup. Or get them new through LMC. If you have the drivers tank open and exposed it would be best to have it cleaned first. Many radiator shops can do this, they put them in a big vat and let them sit overnight to clean them. Rather than finding another pass. tank I would just have the same radiator shop clean it too, then weld up the crack. My local rad. shop charges $75 per tank to boil out, and the crack repair would run $25-50.