Same problem on my 340. I installed a Cloyes double True Roller chain and fought to get it on with the tensioner.
I had to have my brother hold the cam in place so it wouldn't bang against that rear plug. Then, I used my fingers to push against the chain and tensioner at the same time.
Once I got the first row on the tensioner I was able to slowly work the chain and gears into position.
It was tough.