I know this has been covered before, but from what I have searched and found was to either remove the water pump or spend money on aftermarket parts . I found an easy way to install the aftermarket sending unit in the stock location of the oem temp sender on the neck of the water pump on a TII engine swapped into my first gen .
All done in 15 minutes or less. see pics below.

Stock location on the neck of the waterpump
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The pic below is of the oem temp sender with top and bottom cut off. In the middle you will find what looks like plastic, drill this out (only the plastic part) and tap with a 1/8 NPT bit (no need to drill for the NPT tap, the hole is the perfect diameter for the tap).
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Screw in the temperature sending unit.
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Screw fitting back into stock oem location and your done
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Hope this helps

 

i don't know much about GSL-SE's but on 12a the Temperature sender is right beside the oil pressure sender, underneath the oil filter/cooler. i believe what you have there is a TII goose neck if i'm not mistaking.

 

I forgot to mention the engine is a TII, swapped into a first gen.

 

been there, done that..its nothing new. i did it to my s4 n/a..although, you might be on to somethere here if you can get the s4 n/a coolant fill neck to adapt to the 12a.

:AA:

 

he mentioned the T2 swap into a 1st gen in his original post.

 

i did mine simular to your but i cut off the angled part of the neck and then had a machine shop weld a bead on and them mill it down a bit to make it look good

 

That looks a little like my hand fabfricated job, except you can put mine on a 12A.

 

i have an 86 pump housing on my 74 13b and there was a threaded hole in it that i used for mine.

 

sorry i overlooked that part.