Due to the length of your project, I would suggest you find a way to fab up a top plate that connects the front and rear of the chassis to cut out flex. I don't think the front and rear braces, in my opinion are not enough to keep the chassis from flexing.
CF Ofna GTP II Extended Chassis Castle/Flux 2200kv 6s
CF Ofna DM-1 Pro CC1717
CF Sportswerks Mayhem ST
CF Losi LST2