I think it would run, but as been said, drive it slowly. With it being a little out of balance, you wouldn't know until you started driving it, I don't think it would be that bad. Yeah, leave the valves, push rods, unless it was a bent one, in that case take it out and lay it on a flat surface and take a hammer and tap it down. You could get it close enough, and leave the lifters in. Cut the rod down to the bearing and leave that in, it would be better as far as the weight is concerned. Heck, take the idea of the guy with the cut down tree limb for a piston, do that for the intake to block off the cylinder. Otherwise what are you going to weld in there?
I was reading something the other night where these guys took a 350 apart and one of the pistons fell apart in half. They didn't know how long it had been that way. Broke clean in half, the two pieces fell on the floor, and it was still running. Heck that's what a V6 is, is a 350 with the two back cylinders cut off. Of course not exactly the same, made that way but is a 350.