You might look into modding a 500 cfm Holley 2300 for the blow through. There are some mods needed, but your not hauling around a second float bowl of fuel that isn't pulling it's weight.
I haven't seen a spread-bore plate for the Offy. Doesn't mean there isn't one. I have a square bore plate on mine.
Clifford makes a plate for square and two barrel applications. Next time I take things apart, I'll see if my Clifford plate really bolts up to the Offy intake like some folks say...
The 500 is a lot of carb for a stocker, but once you get some boost on, it'll work very well. They are easy to richen up for forced induction. It's basically the same process as for the double pumper, but you don't have to mess with the Secondary circuits!
CJ