The front crossmember where the engine and radiator supports mount are different on the frames. You would need to drill out all the rivets in the front crossmembers and swap them in order to put the 235 into the GMC frame. You'll also need to use the Chevy radiator and radiator support to use the Chevy front sheetmetal. You'll see the difference once the engines and front sheetmetal are out.
Check out this link on the Carter website.
Crossmember differenceCredit to Rob English at oldgmctrucks.com
