Cooling fan sensor location

This is on a Dakota, so please bear with me.

Currently, I have a 87 V6 3.9 engine, with 2 pusher fans mounted in front of the radiator. (The way Shelby did in 89)
My fans are set up, so I have one sensor in the top of the intake manifold, and the second sensor is in a 38mm (1.5 in.) cooling sensor adapter. (see example below)
I need a 44mm or 1.75 adapter for my new motor (5.9 magnum) and I cant find anyone who makes one.
I was wondering if it would be ok to use one of the drain plugs in the block?
I want the second fan to only come on when the first fan cannot keep up.
Also, I considered tapping a 1/8 NPT hole in the inlet side of the waterpump.
Any suggestions would be appreciated.

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