[Barnsnake]

Thrust angle is where the rear wheels are pointed relative to the front wheels. The center point between the rear wheels should be heading directly toward the center point between the front wheels. That would be a thrust angle of zero. Otherwise the rear is trying to pass the front under acceleration. This may not be the cause of your problem, but it needs to be right to start with.

I'm not sure if referencing the pinion angle relative to the carburetor mounting flange is a good measurement. Manufacturers have cut all manner of angles on those flanges to compensate for driveline angles and acceleration compensation for the fuel level. If possible, try to get an actual measurement of the tailshaft angle. The machined surfaces on the u-joint yokes are generally parallel with the tailshaft. Failing that, checking the angle of the lower crank pulley would be a good reference if you have room. Check it, roll the motor 180, and check again to make sure the pulley runs true. That angle off vertical would be equivalent to the tailshaft angle off horizontal. With solid links on your rear suspension I would expect one degree down to be more than enough to compensate for any change due to torque reaction.

Keep us posted on what you find.