It has been done and it does improve the appearance.....

For the long runs, you need to include some method of clamping
the tubes to secure them from the effects of vibration. This can be done
using weld-on studs spaced at 2 or 3 ft intervals. Then, you can
use Adel clamps or similar attachments. Otherwise, your system
will flex itself to death. Try to keep the connections on the outside of the frame so you can service them in the future....this means that your brake lines would be one long piece instead of spliced.

Dont run hose inside the frame (obviously). Even braided hose should
be limited to short pieces to connect hard line at the ends.

Your bulkhead fitting idea is also good if you can figure out how to
make all of the connections and then place the boxing plate...it
may take several sets of hands.

You might consider a weep hole at the lowest point so, if a leak did
occur, you would see some evidence before your frame filled up
with gasoline!!

While you are on the hidden agenda, you should also think about any wires that may be needed down that low and do them also....most wiring will
be thru the body but think about it......any transmission wiring, brake switch wiring, VSS wiring, etc?

mike in tucson