When I needed to replace the senders on my Mk2* I sent photos to
www.fillingstation.com in the USA of my originals who were able to match them to the modern equivalent including the 6 hole fitting and 6V. Since you are building new tanks you could go for any suitable 6V sender which they may be able to supply. Unfortunately I cannot find the invoice and I have tried contacting them before to get the part number for someone else but they do not keep records of customers that long. Could be worth trying them.