This is going to be harder then you think as the Nailhead was never fitted into the 68+ A-body. This means there are no mounts or frame pads available to make it a direct swap (like it will in the 64-67 A-body). You will need to fabricate a mounting system for the engine loosely based on the 65-66 GS Skylark. The driveshaft will need to be shortened and balanced with a ST 400 yoke. You will need stronger front springs. You will need to move and extend the wiring (starter and battery on opposite sides). You will need exhaust manifolds (possibly 65-66 GS will fit). Your radiator in the Skylark has inlet/outlets on the wrong sides for a Nailhead. You will need a rear sump oil pan and rear sump pick-up tube (difficult to come by). Those are the first things that come to mind. I am sure others will jump in with more. It would be a very neat project, but there is more to it then you think at a glance.