CarQuest has a small Engine Temp sending unit: p/n 58079 Standard Part No: TS-17 (63-89 Dodge Truck) 100-220 degrees
Lift the intake off the block, drill and tap a 1/8" pipe-threaded hole in the top-right. Use a 5/16" drill bit to make the hole, then tap with a 1/8" pipe thread tap.
Use some grease on the drill and tap bits to keep the shavings out of the water pump as much as possible.
Here is the sending unit after it's been mounted. Note that we kept it towards the back so that it will avoid the thicker casting on the inside of the block.
Here is a shot of the water jacket that we are drilling to get access to. Note the factory Engine Temp fitting on the Driver's Side.