As I understand it the actual Morel Solid Roller uses the same Hydraulic roller body which are rather long compared to other solid rollers and take shorter pushrods. But functionally they work great and are very durable, and obviosly if they use them for solid applications they are certainly proven at higher than 6000 RPM crank speeds.

If there's any peace of mind remember that 6000 rpm at the crank is proportionally only 3000 RPM at the cam.

I have a nearly new set of Morel SB Mopar solids I only switched them because I wanted to try out the BAM lifters, no difference in performance at all though. I think the only difference is the SB have a V bend in the link bar for valley clearence, I cant think of any reason they wouldn't work in a Big Block.