There's no reason to have a side oiler block, other than being able to say you have a side oiler block. That is, unless you plan to run your car at LeMans for 12 or 24hrs straight. And even then,
oil system improvements and better parts tecnology have improved these motors vastly over the past 40 years.
You aren't just paying for Shelby's name on the Shelby block. It is, without a doubt, the strongest FE block you can buy. With it's main design, and valley webbing layout, it's the top dog of FE blocks.
The other aftermarket blocks are great also. While they don't have the overall block strength as the CS block, honestly, who really needs it? The Genesis or Pond blocks should easily handle more power than anyone could ever put to use in a Cobra.
If I were going to buy an aftermarket FE block (and someday I will), it would be an aluminum Pond block. They have had the least issues, and have the most accurate casting to an original side oiler block.