I don't think that the engine swap will have much impact on your car's value. The 250 engine was installed in the 250SL so it's really not a huge deal. I prefer the manual as well but well known pagoda restorer Gernold Nisius of SL-Tech in Arundel, ME feels that the automatic transmission is better suited for these cars. These are not super high value automobiles so do the swap if you want. If it were a low mileage, well maintained survivor, I'd consider keeping it stock.