You dont want a roller. The whole point, well maybe not the whole point, but a big bonus to a 351 is the strength of it. Just invest in the good lifters to convert it to roller and run a 69-74 block. Those are the strongest you can get.
It's debatable whether or not the '69 blocks are THE strongest. They are one of the strongest for sure, but it seems that the '70-'74 blocks have stayed in one piece at the same horsepower levels and over extended periods of time.....just like the '69's have.
Nate - from what I understand, the '69 blocks have a shorter deck height by 0.01" or so. It's almost negligible because if the piston is out of the hole by a few hundredths, it can be milled off to get the deck back to zero.
As for the differences, from what I've seen, it's in the main webbing. The webbing in the early blocks is much more beefy than in the roller blocks...
I didn't do the search for the 69 Block. I just got the 1996 Block so I didn't have to spend big bucks on the lifters. worst part is I still did Couldn't help it.