Is it possible to lay on top of the engine? I have a friend with an Ericson 26-2 (same interior as your 27) with a 1GM Yanmar. We put a pad on top of the engine and dive in there head first. We also made the stbd quarter berth riser removable for excellent access.
If your coupler is rusted in place, I have a coupler splitter available that can be in your hands in a couple of days via USPS Priority Mail. Pictures of it in use on the same Ericson 26-2
found here.