>>>*Since you don't mention rough running or hesitations, or other types of operational concerns, here are some tips.

An RV type camshaft combined with oversized tires will create a lower RPM softer than normal feel.

Add in a freer flowing exhaust and it gets worse since RPM, thus velocity, is lower than it should be.

An engine that is cold is also richer and this would aggravate it. Another possible is late valve timing from the head/block perhaps having been surfaced?

A combination like that really isn't fun to drive plus it is rough on mileage.

The real solution is gearing if you haven't already, next would be to invest in an adjustable top gear and advance the cam a degree or two.....*EB