THE DISAPPEARING '49 OLDS (and every other model that someone wants but has never been massed produced): Quit crying because RC/ERTL (or whoever they are THIS month) reneged on the '49 Olds. I've found TWO and I'm slow and dense. Hendrix casts them in two body styles and there are those hideous NFL paint scheme racecar/banks out there collecting dust on shelves. Seek and ye shall find.
But OK, it's not the '49 Olds you want to build but something else that's never been made by the styrene mass producers. Don't give up, bunkie, you can still get it done! A little hint.... hit the resin guys. And if that doesn't work, dig through the mass of die cast offerings. Die cast only remains die cast until you strip one down for the body and add enough styrene and resin to make it a REAL scale car! If you want to build something that doesn't exist, do some research, find what applicable parts you can and start filing and sawing and scrounging until you get what you want!
If our hobby was easy the unwashed (and those that build military stuff) would have taken it over long ago! Actually, I'm kidding about the military guys... most of them can do research so thoroughly it'll make your eyes cross. If you know one, approach him (cautiously) and and have him fill you in on researching a project.
If you want to build something that doesn't exist, do some research, find what applicable parts you can and start filing and sawing and scrounging until you get what you want!