David, Many thanks for the link to Dynalite. I have had a good look at the site and sent them an enquiry with a few questions. It looks like a straightforward swap and avoids a negative earth conversion but there may be a need to upgrade some battery wiring and do something regarding the ammeter which may not be able to measure the maximum output. Based on their website, they are expensive as you say but given that the electrical domain in Sunbeams is a somewhat unreliable weak point it may be worth it, as I am doing a major restoration.
Re Joe Parlanti’s article you may have the correct one, though Gary Corbett may be able to confirm this. It is a very different installation on a Tiger than an Alpine.