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

That little bit of rust..... meh....... Get the crank out of the way and knock them out through the bottom. Spray the cylinders with wd40 and they will come out.....

I have pounded pistons out of motors that sat opened in the pouring rain for who knows how long and the machine shop I used never had any problems. No matter what you do an oversize piston is in the future anyway....

Good luck,
JW