here it is 

I have owned this 260 since 1979. I restored it in the early to mid 90s and bought parts for the last 30 years . but for the life of me I can not remember where I picked up this cover , but it sure looks handy for this application . like I said I believed it was tap for 1/4" bspt and I retaped it for 1/4" npt.

by the way like my rotor made in china ! it is a Beck Arnley part! just noticed it in the photo lol

I actually found a thermostat housing like the one @kully 560has - https://maseraticompound.com/products/datsun-240z-thermostat-housing