For Trade Pentax 6x7 600mm f/4 SMC Takumar for SMC Pentax 6X7 500mm f/5.6

I have a Super-Multi-Coated 600mm f/4 Takumar Pentax 6x7 that is longer than I need and wider than I want. (The barrel and cap show wear but the glass is in good shape; no case.) I would like to trade for the smaller SMC Pentax 6X7 500mm f/5.6, likewise in good shape. Thanks, Jack