I'd suggest you get a second flatbed kit and use the 2 kits to make your stretch flatbed, with enough parts leftover to make a second trailer.
You can never have enough trailers!
I've built 3 trailers out of 3 flatbed kits;
a 48 ft. flatbed (lift axle has been removed since pic taken)
a 24 ft. flatbed pup (makes a Rocky Mountain Doubles set)
and a 24 ft. container chassis
