I have some McM A-5 adj, one with the thumbwheel, one with the integral, 4 with the KMW hardware (also have Manners T4A with the KMW).
IMO, the KMW is the way to go, but you'll need to specify when ordering your stock from McMillan that the cheekrest needs to be a KMW inlet, and hardware delete. And then send your stock when you get it to KMW for the cheekrest hardware installation.
Or, I think you can order the Manners T4A which feels very similar to the A-5, with the KMW cheekrest hardware already installed.
IMO, unless there is some specific reason you want the McMillan, the Manners is a very good stock, and you can get the Manners with an integral bedding block, plus the adj cheekrest.
If you are having your build done by one of the 'smiths used by several of the Hide members (GAP, SAC, Jon Beanland, etc.), they could suggest the best way to get what you want.