Is this a 260, 289, or later 302 engine. Some of the intake manifolds for small block Fords have a port behind the distributor and then another one farther left of center closer to the left cylinder head. I'm not sure on the older ones though. Several of the different aftermarket aluminum manifolds have the port on the left. To machine one there would require removal of the manifold. There are block drain plugs that may be used but they are lower in the block and closer to where the coolant inters the block so the temp would be lower than if it was placed closer to the exit of the block. An exception might be if there is a drain at the back of the block but it still may be cooler than the outlet port area. In line hose adapters are available to fit radiator outlet hoses but the unit wouldn't get temp up until the thermostat opens.
The non-electric mechanical reading gauges usually have a relatively long bulb and many need a flared seat to seal correctly. Some are odd size threads too other than 3/8 NPT like 5/8 X 18. What thread does your sender bulb have? Adapters are made to adapt to 3/8" NPT.