Steveo , either block will work well for you for the street.
The 010 casting 2 bolt blocks have a higher nickel alloy and will allow for more twist than the 020 castings.
With the cams available for the street out there now you won't be twisting ( with any power ) over 6500 rpm anyway. But if you are going to build a solid lifter strip motor thats going to turn up towards 10k than it may be an issue, and I would then go with the 010 block & have it splayed to a 4 bolt so that you can feel safe with that Blower, or 250 NoS shot as far as the lower end goes anyway....
Good Luck