She's got potential, but I'd be worried since it's been sitting in the grass and leaves for that long if there's a frame and floor left under it.
IMO, if it's clean underneath, see if you can get the original I6 to run again. Pull the plugs, spray some WD40 down the holes, and try to spin the motor over with a breaker bar and socket on the crank bolt. If it turns easy enough, check over the fuel lines, put some fresh gas and new plugs, wires, and points (I guess it's got points, no HEI in '66) in it and see if she'll run.
Good luck!