I'm thinking of using two longs to hold the data, even if that makes calculations interesting. UtilityBase functions may be suitable for that, otherwise I'll roll my own. This way existing code will continue to work as it does today (little-endian style) and new code can be adapted to see the higher bytes.
Create small structure with two longs and develop small framework around it to do 64-bit math and ToString() function to convert numbers to ASCII.