" So, what I have is a fully rebuilt 261"
"This is a Canadian 261, so the hydraulics are OEM"

It would be divine luck to find new OEM hydraulic lifters, so are your lifters used originals that have been resurfaced or new reproduction?
Which leads to the camshaft. Reconditioned or new reproduction?

I hope this discussion doesn't spin out of control into a regurgitation about lifter design and cam specs.
It all been discussed here before and can be found by searching Stovebolt. Especially noisy hydraulic lifters.

Back on track now. I'm curious. Why do pushrods rotate? Maybe it's because the lifters also rotate?
I'm not a engine man like Jerry but I recall that you want the lifter to rotate a little bit for lubrication and to have and even wear.

"Adding CFM to a truck will only help at engine speeds you don't want to use."
"I found there was nothing to gain beyond 400 CFM."