3.3 revo front and rear center shafts are the same lengths. The 5456 will work perfectly. Keep in mind that even though you have the gmaxx single speed conversion, the tranny shaft is still the same lenght and 6mm in diameter. Just make sure you also buy the 5458x yokes.
Castle Neu 1520 on 6S LiPo Powered Gmaxx (Nitro Killer)
Predator with OS .21TM
Supermaxx with Mach .26
Revo with OS .18TZ
Kyosho ST-RR Conversion
Ofna CR with Tekin ESC/Motor (2)