If you do use one piece, one sided mold, I advise still covering the open back with something after pouring the resin. The mixed resin sets by heat, generated by the mix of chemicals. Any air-exposed surfaces do not get as hot as the inside, and may remain tacky virtually forever. I cover any single sided molds with a little block of particle board while it sets up, to prevent this. Fill to completely full, maybe even oozing over a bit. Push the block down hard to level the excess, and then let it sit for hardening.
BTW, I am contemplating a scratchbuild on one of the big contemporary Diesels, latest monsters, and do anticipate casting the truck sideframes too. Wheels also, as seperate mold.