Thanks for the input!
@adelorenzo: sadly, when I sold the russian lenses, I sold the adapters as well. But I will watch out for another one on ebay and give the 80mm a try.
@carpents: nice shots! I have the 45mm FA as well, very nice lens. Better than its reputation, in my opinion. Funny that you mention the 67 105/2.4. About a week ago, an ebay buyer visited be to pick up an article I sold. He brought along his Pentax 67 together with his 105/2.4. As I already had the Pentax 67 adapter (bought by chance some weeks before), I asked him for a try out. I just took two shots out of my window, one with the 105/2.4 and one with my 645 75/2.8. I expect to be able to pick up the developed film today after work. Perspective was clearly more narrow, but if bokeh and sharpness deliver, I will gladly give it a try.
@pentaxuser: OK, I'll visit the pentax forum, good suggestion, thanks!