I just responded to a precious post, but I also upgraded to the 5765 and 645mg into xt. I also upgraded the radio to a Futaba 3pl. The stock 6v battery doesn't do the job to turn the 5765 well enough. It was struggling to make a nice solid turn even with the servo saver tight and the DR set high.
As soon as I dropped in the 7.2v stick pack, I was able to loosen the saver up and set my end points to about 85%. It makes nice right turns now. I contacted Hitec as the 645mg is rated for 6v, but they said it will be ok.
My concern is the receiver is rated for 6v, not sure if I should upgrade the receiver now. I am getting a slight high pitched whine when I turn everything on. I think it actually may be coming from the t/b servo.
Also, the t/b servo drops right in, you will use the double arm saver that comes with the servo. You have have to make 2 of the holes on the horn slightly bigger do the brake linkage to slip into and for the throttle linkage to screw into.
For the steering servo, you will use the existing hardware to directly bolt in the bottom of top set of holes. You will have to drill 2 new holes for the other set. It's very very easy, no worries. You will use the large white horn that comes with that also and you have to slightly drill out one of the holes. Or you can get an aluminum horn from DDM.
All in all, taking my time, it took less than a half hour to install both.
Hope this helps!