Many of us have ageing 12 gallon tanks and the sender unit is usually the first part to fail. The alloy housing usually corrodes through and leaks, other problems are that the fuel pipe comes loose and rotates as well as leaking at the joint and the electrical connector corrodes away. New ones are unobtainable and NOS ones are very rare, the only good secondhand ones you are likely to find are already fitted to good cars! So the only solution is replacement with an alternative unit.
Possible candidates with the same 6 bolt pattern and PCD are senders from Mini vans, early MG Midgets, some MGB's and the OHV Viva 8 gallon unit. Some Mini, Midget and MGB ones are secured using a compression plate and lock ring, these units are not so suitable as you have to drill the mounting holes. The cork gaskets from Minis etc with the six bolt sender are almost direct replacement for the original gasket, just a single V shaped nick needs to be cut to accommodate the sender workings.
The original 12 gallon and 8 gallon Vauxhall units both operate in the 0-200 ohm range, 0 ohms being full, The Mini and MG units operate in a similar range, one source quotes 20-220 ohms while another quotes 0-200 ohms. All of the above units do not have the 'pass through' fuel supply pipe so that is the first thing to sort out, thankfully 'Chopper' has done just that to a OHV 8 gallon sender unit and here's a step by step guide which could also be followed using a brand new unit from a Mini van etc.
You will need a length of 1/4 inch copper tube, you can use 6mm if you wish, a 6mm (metric) compression bulkhead connector or tank fitting like the one in the photo below, imperial ones are too big to fit inside the opening in the tank, a blowtorch and solder (optional but much better) and a couple of suitable copper washers.
First off you need to check which way is going to be 'up' on the replacement unit, do this by checking the resistance relative to float position, or just work out where the float should be in the original sender unit application, be it Viva, Mini etc.
Secondly you will have to bend the float arm to suit the 12 gallon tank. You should mimic the bends on the original sender unit.
Next, decide where the most suitable place to drill the hole in the alloy unit is, you need to accommodate a bit of space for plumbers compression tank fitting within the access hole in the tank.
You'll need to drill out the metric fitting to 6.5mm to accommodate the imperial 1/4" pipe and pass the pipe through the fitting with suitable lengths on both sides, to get to the bottom of the tank inside and attach the fuel pipe on the outside of the tank.
If you're going to fit the pipe without soldering then you need to fit the connector to the sender and then fit the pipe with olives and compression nuts and tighten everything up.
If soldering then solder the pipe in place before fitting to the sender unit. Keep the pipe straight until all the soldering is done and the connector fitted to the sender body.
Use the copper washers between the alloy body and the brass fixing nuts, bedding them in some petrol resistant sealant.
Tip - keep the copper pipe straight throughout initial assembly - only bend the pick-up pipe after you have locked the back-nut securely in place so that it is not fouling the cork gasket/etc.
The following photos show the final result of fitting a soldered in pipe to a OHV Viva 8 gallon sender and hopefully help explain the text.
Thanks Chopper for the photos of you modified 8 gallon OHV Viva tank sender unit and your assembly tips.