The 283 and 350 (as well as all other smallblocks Chevies) are the same size externally. As to the fitment, I don't think it will line up on the front, you'll probably have to use a side mount setup with a crossmember. Also, the V8 will not bolt up to the bellhousing.
I would advise going with the 350, if only for the increased power and reduced building costs, unless you already have the 283.
