Access fuel tank sender in Kombi

I have a 72 Kombi and would like to cut an access panel into the rear shelf to get to the fuel sender. Does anyone know of the correct dimensions?

Thanks

I followed this link,the only thing i found was the hole could have been about 25mm closer to the front
http://www.specialpatrolgroup.co.uk/spooky/access/access.html

Don't butcher it. Take it to Klaack Motors in Fairfield if you want to see how its done. They replaced the sender in my old kombi (model year 1976) just by taking off the firewall.

you will find a 71 has a different sender to the 76 and with it having the upright motor its just not easy to take out the dividing firewall,its either cut an access hole or motor out, then tank out ,been there done that with my 73 with the long tube type sender