why?
With the 426 rivets you already have a dimple and/or countersink that could be difficult to make larger, and is probably visible when the plane is done. But with a 470 you should be able to just expand the hole and use the next larger diameter rivet without any issues.
The manufactured head would be visibly larger than the surrounding rivets. I'm not sure how noticeably different the shop head would be. However, there are not too many places in RVs where a line of 470 rivet heads is visible after you are finished building.
Am I missing something here?