Balancing engines is not really a science in the sense that you have a perfectly balanced rotating assembly, there are calculations made that take into account projected weights of pistons at speed and other imaginary numbers. That being said as long as your new balancer has the same external balance pad as your old 69 balancer then you can go ahead and swap them. As near as I know all 351W are externally balanced and this would mean all of them would have the same style of balancer. They never drill holes or put Mallory metal into a balancer anyway. The problem now becomes one in which should you use the newer flex plate or flywheel also to have a matched assembly, I would just to be on the safe side.
“She may not look like much, but she's got it where it counts, kid. I've made a lot of special modifications myself.”
— Han Solo