Yeah, lots of cam problems since they removed most of the zinc(I think it was?) from most modern motor oils. Roller cams don't need it and it's a component of air pollution, so it's mostly bye-bye.
I would start adding a zinc component or use oil specifically stated to have it, or be compatible with "all cams" or something similar. Castrol's great stuff, but I'm sure some of their stuff is not going to be right for your app. At the very least, read the can.
If nothing you do or check works, take a valve cover off and measure some lobe lifts. Kind of a pain, for sure, but if nothing else pans out, it's something worth thinking about. You would think you'd notice a big drop in power, but if it's only one or two lobes, and the engine is pretty healthy, you might not notice right away.
Carb is fine. 600 is not too big even for a bone stock engine. It's borderline unnecessary at that point, but it's not so bad as to make it run poorly. You'd simply feel a slight improvement with a smaller carb.
With even a mildly better-than-stock engine however, that 600 should be perfect.
Not sure what diff the 351 heads make, good or bad, other than (possibly) having better breathing characteristics with slightly larger ports and/or a better combustion chamber shape. But that might all depend on what year and model 351 was the donor.
Good luck.
Paul