Dont spend money on roller rockers. They wont do anything for performance on a street build.
The 78 block is most likely the 557 block. It will be an OK block, but if your building in excess of 400 horses the older block would be better.
If it were mine, I would sell the 557 as a buildable core, and use the money to get what is needed to put the other one in the car.
Fact is stranger than Fiction