The tensioner blocks that rub the chain were made for narrow single width chains. A good doube roller is plenty strong. Just my opinion that they aren't necessary. I put one on my 273 when I built it but I wouldn't do it again.
So you are saying the plastic is not wide enough for a double roller.
I did not think strength had anything to do with it. I thought it was to keep the slack that all chain eventually get, from disrupting the cam timing.