The only reason I can see for getting another block is if you want to get a 302 roller block. Or if the one you have has been bored over too far. I think the older blocks are actually a little stronger as well.
Assuming you have a 289 - yes. You essentially have a roller 289.
* You will have to plug the oil dipstick passage on the side of the block
* You will need to use steel distributor gear for the roller cam
* Firing order is different