What should I do with my 340 that has been sitting for years.

A wire wheel on a drill will knock most of that rust out but I would see if you can pop the crank out and leave the rods (with caps on) and pistons in the cylinders then push the pistons down and wire wheel the rust out and pop out the pistons. I have used a chunk of 2x4 and a sledge hammer to tap them out.