Hmmm, I don't know about that, you can try this:
Yaesu FT-7900R MARS/CAP Mod
Even if you were able to freeband this mobile rig, I sincerely would suggest against prolonged (or any for that matter) transmissions at or around the frequencies of 478MHz. This radio was designed, tuned, and tweaked for transmitting in the specified ham band and sure it could work slight above and below that but at 28MHz above the intended upper limit, you're asking for it. I'd find a cheap high-split commercial UHF radio instead, just a suggestion.