Bryson - Donal Logue, Nick Offerman, John C. Reilly, or Paul Giamatti
Katz - Dan Fogler, Zack Galifianakis, Kevin Heffernan, Kevin Smith, Vincent D'nofrio, or Val Kilmer
I know some picks are out of left field but please dont ruin this movie by casting older men!! If they want to make a movie about older men finding themselves, I'm sure there are plenty of books like that waiting to be adapted. Don't ruin one of my all time favorites!
I can see Paul Giamatti as Bryson and Zac Galiafanakis as Katz could be hilarious.
Luna
Since Dom DeLuise is no longer with us, I agree on John C. Reilly as Bryson, but he doesn't seem... I don't know, "refined" enough. Maybe Nick Offerman.
But for Katz, there was one actor I had in mind even when I was reading the book years ago: Brad Garrett.
Actually, reading through your Katz choices, Vincent D'Onofrio would make a good Bryson.
Really?
Two threads on casting?
Get over yourself.
Perhaps the OP just wants to reach out for some sense of community.