Harvesting the sweet spot... hoping to get the 67-K adapter

I am very amazed at the limited information on this accessory, the Pentax 6x7/67 lens to K-mount adapter.  They are out there as many sources from China would show and so I would assume a demand exists, but no real talk about which are the good ones or bad or even if they are all the same.  Maybe the lack of info would be possibly an unwilling admission of foolishness of being duped into buying a piece of junk.  I've run across plenty of chatter people who "have tried one out" or "used one for a bit" and as commonly the expression of confusion or contempt as to "why anyone would bother".  My quest for one now is beyond the curiosity state and now in the need it now stage.

"So, what the project this time?"
Having an extremely difficult low light situation coming up and the need for sharpness at a distance of about 150 feet and beyond, I find my usual "go-to" lenses are not cutting it for me.  So what I have, I can't run wide open and what I own that might work, does not fit.  So, all those freshly acquired Pentax 67 lenses in the past several months I have two that may solve my distance and low light needs.  The candidates are the 165mm 2.8 and the 105 2.4.  Both offer a sharpness over 60 lpmm at center while wide open.  Most lenses are around 42 lpmm, if you are lucky, with a center coverage field of 20-40% on an APS-C sensor.  However, a 6x7's center coverage on a APS-C sensor is going to deliver a more complete coverage and at the higher resolution even wide open or so I hope.  I examined what few tests sources of my own films and they show that the sharpness diameter of the 6x7 lens is about 2-4 centimeters prior to stopping-down or in other words the coverage with the highest possible inherent sharpness is covering 90% of the APS-C surface.  I am hopeful and wouldn't know for sure until I try this out.