You can somewhat convert a 7.5 into a 777 since they share a lot of plastic. You'd need the shock towers, front and rear arms, front hub carriers, front upper arms and pin blocks, possibly the steering ackerman plate, rear squat plates and toe blocks, and possibly different length shocks depending on what model 7.5 you have. Those parts will get you very close to having a complete 777, only other major things needed would be the chassis and radio tray, again depending on what model 7.5 you have; a Kanai 2 or 3 would get you some aluminum bling that came on the SP1/SP2 cars, but is not really necessary. I went back to plastic for a lot of these parts on my Kanai 2.5 to reduce weight, as a matter of fact.
EDIT: buying all of this stuff new won't be cheap, so you may be better off adding 777 parts as needed. I'm running 777 front arms and hub carriers because they allow more suspension travel and for the added adjustment the 777 hub carriers provide.