How's the belt? It looks like it's riding pretty low in the clutch. If it's riding on the bottom of the groove instead of the sides it will slip. I put a new belt on an old clutch I have and it sat about 1/16 below the outside of the clutch.
I fought with my Miller for a long time, seemed to had a wire feed problem. New rolls, liner, tips and nozzles didn't help. Then I ran out of wire and got a new spool. It feeds like a new machine now. Just a crappy spool of wire.
Were both pistons you tried new aftermarket? It could be the pin holes aren't bored square to the piston. I would have thought bent rod but you tried another with the same result. Were the rub marks on the rod at the top on one side and bottom on the other?
I read through this again. You are using the old crank and rod but you didn't say if you have a new piston. When you put it together in the block, look at the piston end of the rod and see if there is about equal clearance on both sides of the rod to the piston bosses. I think the rod is getting...
Grew up helping Dad when he ran his Monkey Ward saw just like that one. That's 130-some cc's of cast iron Tecumseh and they have a sound all their own. I still have it and the last time I ran it a few years ago the sound of it took me right back to being a kid dragging branches out of the way...