They pushed the flow up to 90 cfs yesterday, so the fishing should be pretty good. You might want to try soft hackles instead of nymphs (or not) but other than that you plan sounds as good as any. My personal preference in terrestrials are parachute ants. A size 16 was working pretty well for me before last weekend's deluge.
There might be tricos in the morning, but they start later in the year on the Gunpowder than any other river I know about.
The "cold" is a relative term. We're talking mid-50's, not winter type temps. It's probably warmer than Big Spring is at the moment. It will fish just fine any time of day; the biggest concern is probably not spooking fish. Use a long leader, and pay attention to shadows/shade.