Robert,
I had a thought if you are going to Paint in USAF scheme you will need a fictitious serial number to put on the tail. In the past the USAF for display purposes has used the Year built plus the build Number. For your plane it would be "681293" or you could use the "614376" the number that would have been used on the last incomplete U-10D that became 1401 (the next Helio built after Yours), you could use 8 for the first # because 8 would be the abbreviation for your build year '68.
What do you think or have you already picked something out?
Doug