The '68 and '69 Camaro Z28 302 motors used a crank with the larger (2.45"/2.10") journals. It should bolt right into your 350 block. Casting number on the '68 crank is 3923279, '69 is 3941178.

Scooter has done a bunch of those bored out 283 blocks, but you'd be much better off in my opinion to use a Z28 crank in a 350 block. The cylinder walls wouldn't be so thin.

Like I said, call some of the Chevy parts departments and see if they just might have one on the shelf. Also do a search for the crank on Google.