Strange exhaust issue -'63 R1

[Hogtrough]

My '63 R1 Avanti has distinctly lower pressure from the driver side exhaust pipe - probably a third of the velocity of the passenger side. I checked Youtube and found various ways of checking for exhaust restrictions, including using a thermal imagining camera . Don't have one, but used my trusty laser thermometer instead. The results were rather dramatic: Driver side muffler 104 degrees, can easily hold my hand on it. Passenger side over 300 degrees, hot stove territory. This is a fully rebuilt engine that runs (in my opinion) very strong. What could cause this?