Thanks guys. It look like I'm a little too far past 6oclock position where both your photos seem to indicate. The new sender didn't come with a new locking ring therefore I did use the old one but put a bit of grease on the back of it to try and stop it grabbing the sender but I did see it turn past. I shall try again
That's odd that the new sender unit did not come with a lock ring and new gasket. What brand did you get?
I checked Rock Auto for 71-73 Mustang sending units and they are again listing an INCORRECT part for our cars. They list Spectra Premium FG87A. This is the one I originally bought and the tube faces the WRONG WAY!!! In my case I was able to return it and exchange for the Dorman 692232, which is stainless steel, brass float. Unfortunately they no longer list this one. The only other one is SS, but out o stock. However they are supplied with lock rings and gaskets as yours should have been.
I would make sure the one you bought is stainless steel, brass float with a good filter screen. If not take it back and buy a better one. The Ford part number is D1ZF-9275-AB (on the one I have). You will likely need to calibrate it as well. Use 5 gallons in an empty tank and adjust the float so it reads a 1/4 tank. Mine only showed 1/8th, so I needed to bend the float arm slightly downward. The trick is to jack up the left side until the sender can be removed without spilling gas all over the place. Then recheck it. It's a PITA,, but I like to know when I'm getting to a 1/4 tank.
Here's a pic of the wrong part, just for reference. I have no idea what this is actually for, but as most Ford's have the fuel lines on the driver side, it is either for a tank with a center location or on the right hand side. I'm showing this so others won't buy this part. I'll also contact Rock Auto and point this out again!