First you cannot tell a side oiler from just part numbers. The marine or industrial blocks used the same castings. The blocks were drilled as top oilers so you will have to look.
1. All 427 haveCross bolt mains, look for bolts just above the pan rails
2. All side oilers had screw in Freeze plugs, but then again so did some industrial and marine blocks
3. Side oiler blocks have plugs in the side of the block where the "side oiling"
oil passages were drilled.
Something else to keep in mind is that service (ribbed) blocks are worth less money due to core shift problems. The early blocks are also much harder to find. Ask Computer Works how long he has looked.