Make sure the t-stat isn't failed open. Had that exact same issue on my 87 ShelbyZ even with a bunch of aero cooling mods to it. When it fails open the coolant doesn't stay in the rad long enough to cool off and the temps start climbing at 65MPH cruise.
As for replacing the factory "air fence" under the rad go to the j-yard, find an 80's to early 90's Chevy S10 without front tow hooks and pull the air dam off it. Take the bolts and everything. Then use those bolts to bolt it to the bottom of the rad support, and mount the corners to the corners of the factory air dam by the wheels. This is pretty much the way the 84-86 turbo Z's were. It helps cooling and front end down-force.
very old pic but gives you the idea
Minivan but tucked up in the corners instead of pulled out like it should be.