I haven't done a tube shock swap on my '51 Special, but to do that, you will need custom mounts made and welded in. Before you go to that trouble, you may want to see if your lever shocks can be revived. The fluid has probably leaked out of inoperative shocks. There is a filler plug on top of the shock that unscrews. Hydraulic jack oil should be used. If the seals aren't too bad, there is a chance you may be able to salvage them. Rebuilt lever shocks are available.