Could someone tell me the difference( different switches,etc.) between a 71 thermostat housing and a later 81ZX housing. I've looked, I really can't find any info. I see the 71 has 2 bolts to hold coolant outlet on. ZX has 3. Thank you very much.

Where have you looked so far? No use going over ground you've already covered.

The switches are used for either Emissions or Engine Fuel control. Look through the FSM's in those chapters and you'll probably find what you need.

just the housing itself, or the purpose of all the switches & sensors?

The housing itself is a different casting, the late ones have three bolts and a bunch of tapped holes for sensors, the early ones have two bolts and less tapped holes for sensors, as you've probably already found. They both will bolt to any L block as long as you have the matching housing & cover.