I think 66/16 is too tall. I'm running 51/20 with UE 8 spyders and still have plenty of power to flip my extended(monoblock) gmaxx.
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)