Sure you can. You'd just need to drill/cut a length in the mount holes and countersink the longer hole. To make the countersink as good as possible, I reckon it'd be easiest using a standard file and do it manually.

Using scriptasylum.com, the theoretical top speed with 66/16 gearing using a 1490kv motor on 4S with 6" wheels is ca. 62kph/38.50mph - this is my current setup. Same specs but with a more "normal" motor such as 2000kv or 2200kv gives 83kph/51.70mph and 91.50kph/57mph respectively. If running on a track, I wouldn't put more than 1800kv in it if you don't make other changes. This would give loads of torque and a top speed of about 75kph/36.50mph.