Hi Bruce
On the later HUPs that hole is one of two mounting studs for the splash shield on earlier trucks this shield was held in by 5 bolts the later ones are a slip in and turn latch.
Yes you can just get the shield out without removing the expansion tank. You turn the latch pull out on the panel and scrape your knuckles. The two mounting post/studs are a source of rattles and squeaks.
My expansion tank is fabricated about 40 years ago still good, and yes it works very good to collect the over flow and have it drawn back into the engine.
Hope these help. Also took some photos of the map light and how the wire is routed if you want them. Along with the wire route for the dome light.
Cheers Phil