I think the driving lights are made to handle higher wattage so the 18s should be fine. The horns and side-markers are a different matter. I've done 9s and 11s in there. If you do a side-marker conversion (connecting an extra set to the indicators so they blink), you'll want to use 11s for the blinker so they will show "above" the 9s while running at night. I haven't changed the rears at all - lazy I guess. I've been thinking about the LED conversion for the rears.

-- John