A lot of people that I know skate 147s (or any 8 inch trucks) on 8.5 boards, so it's not a problem. Plus you can size back down to 8.25s no problem if you keep skating 147s.
Personally, I would recommend sizing up to 149s if you continue skating 8.38s or if you plan to size up to 8.5s, it'll be a bit more stable.