As far as I know, the '57 283 (first year of the 283, since '55 and '56 were all 265s) was the only 283 model to not feature the side motor mounts. I think they started the side mounts in the 1958 cars. For a long time the blocks were drilled and tapped for front mounting tabs as well. I'd say the odds of you having a '57 block are fairly low, but certainly not impossible.
