I used the MP AL rocker hold downs some years ago. The engine is a 496B with comp cast steel alloy rockers. From memory, I used an ARP fastener kit including rocker studs that has worked fine.

I did have to have a little material milled of the sides of the hold downs to create clearance and line up rockers and valves with shims. I also wanted to make sure I had at least one shim between the steel rocker and AL hold down to prevent excess wear. With that it has worked great through some hard miles and one freshen.