I swear I have seen this design, not sure where or for what application, anyone know if someone makes an oil drain plug that is also the oil temp sensor?

A 1/2" NF drain plug, which is the most common drain plug size, can be carefully drilled and tapped to 1/8" NPT, which is a common size for electric temp sender units. I don't know of a vendor that sells them - we always just made our own. Since I have access to a lathe, I always use one to drill the hole to make sure it is centered for maximum strength.

no actual exp on this so just thinking out loud but what about one in one of the oil pipe plugs at the top rear of the (Big) block? If you do rework/add fittings to the bottom drain plug as you know be sure everything is sealed tight.

VDO used to.

If you try to adapt a NPT sender, make sure the tip is fully in the oil, otherwise it will read low.

Easy, drill the plug and tap for pipe thread. Screw the sender in.
Doug