Temp Sending Unit
 

have a '69 912 tub with an '86 3.2l in the back. Not getting temp sent to the gauge, no reading at all. Have been told wrong sending unit and to get one to fit a '75 to '78. Does this make sense to anyone?

You need one calibrated for the gauge.
If the ones from '75 to '78 are the same as the '69 then it makes sense.
Or just use the one for a '69.

'74-76 temp gauge: use 74-76 or SC senders (901.641.632.00)
'77 temp gauge: use '77, 930 or 3.2 senders (911.606.112.00)

-68 use 901.741.632.00
69- use 901.641.632.00

So yes, a 3.2 temperature sender is not matched to a '69 gauge; fit an SC one.

But mismatching an SC or 3.2 sender against 74-76 or 77 gauges only makes them read 30-40 degrees off, it seems unlikely to make it not read at all.