The 283 was front mounted 'til '59 in trucks for sure, possibly later as well. Side mounts came in on cars in '58.
As far as the 350 vs. 283 thing, it really boilds down to parts availability. The 350 is the cheapest engine to build, even less to build than the 283. I won't say the 283 is a bad engine, it isn't, but for overall cost of rebuilding , the 350 is the winner there.
