HELP rebuild my carter or buy new eddie ?????? 340

Your 68 340 should have a 750 AVS if it's the factory carb. It also had 10.5 (advertised) compression. If your factory carb is all there and hasn't been hacked, I would just rebuild it. You may need to have the throttle shafts bushed, but this can be done far cheaper than a new carb.

If you are intent on a new carb, a new 750 AVS would bolt right on and look somewhat stock. If you are one of the Carter/Eddy haters a good old Holley 3310 would also be a good choice. No point in going smaller on the carb, it WON'T help gas mileage.