I would make sure all the solder points are tight especially the one at the front of the barrel/tube where the front sight is. Then I would look carefully at the rifling and push a patch thru from the rear after removing the bolt to make sure there is consistent pressure on the patch meaning there are no loose places in the barrel. I use a variety of pellets in several that I own and they are very accurate. I use the Crosman premiers, JSB's, etc. Lastly I would try some RWS super domes if your barrel is worn because they seem to be a little oversized.
Scoping an old Benjamin requires some method of attaching it to the gun and I see it can be done but I would question the mounting method first then the scope itself. I suggest trying it without the scope to rule that out.
I have trashed a cheaper scope on a modified 1377 and the only thing I could blame was the arm slapping the compression tube.
Other than that the bore size, condition, and soldering of the barrel (as mentioned) is in question.
Neat old guns but groups like that on a good one is not expected.
Since your post implies that it shot fine before the rebuild,
and it shoots equally bad with scope and open sights,
Then...
Is it possible that the rebuild process caused some damage or abnormality in the chamber/throat/transfer port area? Any abnormality in that area (or even the crown for that matter) can damage the pellet as it passes and damage to the pellet can cause accuracy to decrease.
