When I put the new Pelican Wahler 80*C thermostat in my '87 the temp went to half up to 3/4.
Tested this a couple of weeks ago with new 80*C thermostat when I flushed and refilled coolant.
I took out the 80* thermostat and installed a new Wahler 71*C thermostat (which was same as the old temp thermostat in the car at the beginning when I started)
Temp stays at 1/3 to 1/2 with AC on at 100*+ ambient temps. (Same as before when I began the coolant flush and refill.)
Radiator fan relay/switch is 75*C
http://forums.pelicanparts.com/6788775-post4.html
Also, are you using the correct thermostat in your updated WP?
Do you have the "new or old style" upgraded WP?
Thermostat - General Information, Testing, and Replacement
John