Blocking off the return was a good test to prove that the return orifice was too large in diameter.
You could do some testing with a smaller orifice if you wanted to keep the return. If you want to try this, your goal is to keep the hole small enough to keep the fuel pressure steady and still allow the fuel to return to the tank. Once the fuel pressure starts to bounce around, then you know that the orifice is too large. Other members helped me out on this a few weeks ago. It seems that the return hole orifice range that worked was between .050" and .060". There were a couple of members that had their return holes a little larger and a little smaller, but this seemed to be the average orifice size for the return.