Thank you all! Had a little snafu with clearance between the new pulley and water pump yesterday, so many parts came off the front of the engine for measuring, head scratching, swapping and subsequent fettling. The water pump casting is different between my spare engine and the one fitted. The spare cleared the modified crank pulley nicely with room to change the belt, the other sat a couple of mm too low for the pulley to even fit. Annoying, but it's no fun if it's easy.
Made some progress on the mounting plate, which can be seen in the pics below. I'm going for stiffening ribs and a bottom support structure for the gearbox rather than one piece of thick plate:
More to come once I've organised the latest photos and made some meaningful progress on getting everything fitted.