Whether or not the cruise 'surge' is caused by the rods will depend on the engine load & rpm. It varies, but the engine cruising up to 50-60 mph may be on the T slot. The fix for that is to enlarge the IFR in 0.002" steps. Try the new rods first.
I am at a loss as to how it suddenly got so much worse after replacing the lifter (and removing the carb for that job). It was BAD, barely could keep it going 45 MPH up a slight grade. Before this I had put several thousands of miles on it with this carb tuning driving all over Arizona (1000' to 8000') and Southern California at sea level. Everything about this engine has been a mystery started literally between breakfast and dinner on the last day at Spring Fling in June.