1. So did you ever check the thrust line?
That was my first thought is you might have a slight left not right thrust. I would use about 1/8 to 1/4 inch less distance on the starboard side prop tip when horizontally placed. You can measure with string from the rudder post to each side of the prop but keep the tension the same when you do or you will stretch the string and get a false reading. Starboard side must be shorter when sitting on the wheels.

2. consider adding 1/4 to 1/2 oz lead to the starboard wingtip on the under wing side, not visible. This tamed many of my warbirds and I do consistently on most now as just another step and epoxy it inside the wingtip.