re: "...how can I check to see if the oil pump is working properly?..."

First, I'd watch the idiot light to see if it goes off promptly (it should be lit before starting, and go out immediately). Of course, I'll admit that this was pretty obvious, but the next step would be more involved. You could get hold of a mechanical pressure gauge and then get the appropriate adapters to let you install it temporarily in place of the oil pressostat (that's what you call a pressure-activated switch) that's screwed into the block above the oil filter -- only that way will you know for sure what's happening.

Of course, I can agree that you have good reason to be worried, because the seizure of the cam is extremely unusual for these red block engines (I can't remember anyone reporting it on this forum in all the years I've been on it), so low oil is worthy of concern (unless you've done something with the engine that you aren't reporting). But then, I can't remember anyone reporting a failed oil pump either (unless it was used for racing, or other extreme application).

re: "...What could be causing the car to overheat?..."

Normally, aside from your recent head work, I'd say that you first want to confirm that it is truly overheating, because the TCP (temperature compensation board) is absolutely notorious for failing and giving frightening but completely false readings. You can use an IR thermometer (or a contact thermocouple that comes with some VOMs) on the thermostat housing or driver's side tank of the radiator to verify whether it is overheating or not.

Of course, having just replaced the head, I'd doubt the possibility that it's merely coincidence, and strongly suspect that something related to the replacement of the head is responsible. Head gasket?

Notice anything about the exhaust (volume or color of), especially after the first 10 minutes when you say the car starts to act up? If so, try releasing the overflow tank's pressure cap -- maybe coolant pressure, raised by higher temperature, is being forced into the cylinders and burned? Take a look at the spark plugs' tips to see if they're colored funny, or have unusual deposits?

Look into the overflow tank -- is there a scum of oil? Maybe oil is flowing into the coolant.
After the first 10 minutes, is there bubbling? Maybe combustion gases are passing into the coolant channels (and being released into the tank). There is a kit to test the overflow tank for the presence of combustion gases, to check for such a leak.

Summary:
Look for more clues as above, and anything else you notice, and report back.

Good luck.