Other than the front governor, a 390 is about 1/2" wider with slightly bigger cylinder heads and carries about 8lbs over a similar angle valve 360.
The front governor runs at a different gear ratio from the rear governor models which is no real problem other than ordering the correct governor. Stock Vans prop control cable won't work, but another longer standard length worked just fine on my RV8.
Vetterman already supplies a 390 exhaust system, so fitting the 390 to a RV7/8. poses no real problem.
You can talk to Barrett Performance or Mattituck for more engine details...