REPU temp sending unit into S5 engine
 

I am in the middle of setting up an S5 NA engine in a 75 REPU. Most of it is done, but the temp gauge sending unit is hanging me up. As far as I understand this is the one-wire unit under the oil filter in the rear iron, and on the REPU it's an M8x0.75 thread while the S5 is 1/8NPT. I also have read that I need to use the sender from the truck as the S5 sender won't play nice with the REPU gauge. SO I've been looking into adapters between those two sizes, and can't find anything.

Part 2: I looked into using the plugged-off hole in the front iron, which as far as I can see is M16x1.5 (I don't have a thread checker that big, so cannot confirm but have read online that that's the size). I thought it'd be easier to find a larger metric to smaller metric adapter, but cannot find an M16x1.5 to M8x0.75 adapter.

So that's where I'm at yall. I now come to the RX7club looking for wisdom from those who have swapped later blocks into earlier cars. What could I do besides drilling and tapping an M16 plug to receive the sending unit? Am I correct that the REPU unit must be used in order for the gauge to function properly? Are the sizes I'm listing even correct? Help?