Just adding information for your consideration.

Similar to your thoughts, GMC has a breather/oil fill design.
https://www.stovebolt.com/ubbthreads/ubbthreads.php?ubb=showflat&Number=872839

I suspect the GMC valve cover has an oil deflector inside (also used on V8 engines).
Oil slinging off the rockers can be directed at your vacuum hose.
A true professional made oil "catch can" is an excellent idea in any PCV system that has engines with age/wear.
It has to be checked and emptied on a regular basis.

The original specification on the 261 PCV valve, for hose connections, is the AC CV609 (Weatherly Index 660).
The same valve is used on the next generation 230 engines (GM 6421235).
Make sure your FV410 has the same specifications as the CV609.

"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."