Jesse Ventura as Mongo
Lea Salonga as Miss Saigon
Wait- much better choice than Gary Cole for The Waco Kid- and I know he can sing and does musicals-
Scott Bakula.
If you want to go younger, Eric McCormack.
I’ve always said that Blazing Saddles is a movie about geology.
After all, the opening line of the title song goes:
“Erode a blazing saddle…” 
Giants rookie tight end Jeremy Shockey was born to play Mongo.
Denzel Washington as the smooth, sophisticated Bart.
James Woods as Hedley Lamarr… but can he sing? (Maybe he could just sneer his way through it.)
Tim Robbins as the Waco Kid.
Parker Posey as Madeline Kahn (as Marlene Dietrich).
Who would play the racist old lady who has a change of heart and bakes Sheriff Bart a pie? How about a bit of stunt casting… Karen Grassle? (She played “Ma” Ingalls in the “Little House on the Prairie” series.) 
[Whew! I’m… so… tired!]
He’d be greaaaat;)
Seriously, he seems pretty laid back, could be good in this part.
I just finished reading an article on Hall-of-Famer Mike Webster. His son Garrett would make a great Mongo. All 6-foot-9, 340 pounds of him.