XB GT vs XA GT,the differences that matter for authentication

The XA and XB GT share the Cleveland 351 engine but there are important differences that affect authentication. The XA uses the 4V Cleveland with closed-chamber heads and a spread-bore Autolite 4300 carburettor. The XB moved to the 2V Cleveland with open-chamber heads and a Motorcraft 2100 two-barrel. The gearbox changed from the top-loader to the Borg-Warner T-bar automatic on many XB GTs. These differences mean you cannot pass off XB running gear as XA-correct. Check the casting numbers on the heads and the carburettor type against the build sheet.