hello brad, and welcome to the forums!
just loged on and seen your post here. glad to be drawing a few more like us.from reading your post it seems to me that you have the hardest part down,that is the semi frame you would have to use. as for the cab, if you can find a model of the freightliner,your set, but if not, do like lizardqing said, and go for the resin cabs. FREDS CUSTOM MODELS in san antonio texas, does all emergency resin casting. police fire ems. give him a look,he may just have what you need? he runs ads in scale auto mag, i think he charges a buck for a catalog?i used an ambulance body from him and was very impressed with the casting.hope to hear more from you.