I came back from a longer than normal run recently and found the radio/sub amp having issues when I pulled into the driveway which came good when headlights were turned off. The 14V I had across the battery with engine running quickly dropped away to around 12.8V with engine off. Battery is an Optima Yellowtop installed in October.

A quick tally of the current (lowbeams/tails/electric fans) added to around 41Amps without radio and sub considered.

I checked the Delco Remy 1100794 alternator's rating ... it's only 37A!! so the battery was slowly discharging when driving.

Although the Remy recommended replacement (p/n 20169) rated at 61A would probably be ok I am wondering if there are 'standard' upgraded types in use in FGFs which keep the look of the original but have higher output, say 70-80 Amps?

You could replace your externally regulated alternator with a 10SI or12SI [Systems Integrated] internally regulated alternator. The 10si build by Delco only went to 63 amps but I think aftermarket companies such as powermaster wired then to put out 100 amps. They sell a lot of polished shiny ones but you can get natural for a more original look. The 12si were up to 94 amps but I have one wound to put out 140, not that i'll ever need anywhere near that, I don't even have a AM radio.
The 10si alternator is easy to install in your car and looks close to the 10DN externally regulated alternator.
One just wires it with the existing wires in your harness and the external regulator can be left in place to make the car look original if that is important, although with electric fans and radio amps ???
If you are running a large amp alternator you will want to upgrade the size of the wires you are using in your harness.

There may be a company out there that rewinds the original alternators for a higher output, don't know.

Sounds like a junk Optima to me. I would have it checked first. I have several times about bad batteries from them.

I too use a 10si instead a 10DN. Pretty simple upgrade. They make a plug in pigtail to bypass the external voltage regulator or you can buy a new engine harness that is already wired for it.

I forgot to ask where you connect your accessories, such as the speaker amplifier and any other higher amp component? Most manufacturers of such suggest the battery, but I would suggest the splice at the horn relay. Connecting any of these items via a switched or unswitched voltage source under the dash could be asking for trouble as the wires gm put in the harness are light gauge made to carry light loads. A high amp draw through smaller wires will cause heat build up. There is also more resistance in the thinner wires than a thicker wire you would run from under the hood. More resistance more heat, more heat more resistance and so on. Running accessories directly from the main spice with appropriate size wires and circuit protection in the way of fuses, breakers or fusible links is the way to go, imo.

The battery is fully charged at 12.8 volts. The reading you have at the battery of 14+ volts running and 12+ after you shut down is normal.

Just my opinion, I'm not an expert.

Thanks for the info guys, I'll investigate further a change to the 10si or 12si. From a quick look I see there are plenty of options with these, even my favourite Rock Auto has them cheap. In the meantime I'll drive it with the parkers as DRLs rather than headlights... gotta have me tunes.

The accessories are powered from the auxiliary terminals on the side of the battery thru a heavy duty relay activated by the ignition key.

The battery was much lower than where it normally sits so meter probably read <12V not 12.8.

Good idea using a relay , do you have the circuit or circuits protected with fusible links? The battery is connected to the alternator via a 14 gauge fusible link running to the rad support junction block. From what I understand a 14 gauge fusible link has a rating of 16 amps under the hood [170 degrees] and 30 amps in places other than underhood, don't know if that's correct. You're charging the battery and running your accessories through the same fusible link. You will want the accessories protected just in case.

I also agree with Rohrt, charge your battery fully and give it a load test just to be sure it's an alternator snag not a defective battery.