The 283 has a cool factor the 350 never will. I'm gearing up to pull the 283 out of my '57 Chevy to replace it with a 350. Why? Because the 350 will offer more power, similar mileage, and is cheaper to maintain.
Not all small blocks have the side mount bosses, but you're not likely to find a motor without them. Not all small blocks have the front holes drilled and tapped, though I think most do.
You're biggest problem is exhaust/steering box interference.
