I am pretty sure the y pipe from an 01 2500hd would fit fine, I cut mine up to make my turbo set up but remember holding it up in the stock spot thinking how easy a N/A build would of been.
I just swapped a turbo 6.0 into a 99 c2500. The stock exhaust manifolds fit perfectly using the dirty dingo mounts with the trans in the stock location. I'm not sure if they bolt up to the factory y pipe but I believe you can make it work pretty easily or use the y pipe from the donor...