Many years ago I asked about using an underdrive crank pulley on the street to free up a few extra HP. As I recall, there was a lot of mixed feedback on this.

My car has a 26" radiator, OEM shroud, MP clutch fan, a high-volume water pump and 160* thermostat.

With the tire & gear combination I use, highway cruise RPM is 3000++; idle speed is about 1200.

Never had cooling or charging problems in the past using an OEM alternator, and the only real electrical draws are the Crane HI-6 ignition and Mallory 140 fuel pump.

Is it something that could be worth trying, or am I setting myself up for cooling and/or charging issues for the sake of maybe 10 HP?

With your high RM cruising speed you wouldn't even be slowing the accessories down to what a modern system would have.
I'd try it.

I run an electric water pump but I have had zero charging issues with a 5" pulley on my car. GM CS130 alternator, two electric fuel pumps, electric water pump, two electric fans, fuel cooler fan, Fuel Injection, street car. It can still keep up with the lights and wipers on as well.

I'm using one. The nicer dark blue billet style they discontinued (they didn't
make them for very long...they're not easy to find...so I collect them, so far have only found two).

You will probably be fine as long as you don't experiment with additional electric accessories.

The charging system will be at it's limit with what little draw it has. Idle with the headlights on is pushing it.

I use the same 5 blade fan, but went to a much tighter RV clutch due to low speed cooling concerns.
(not verified problems...just concerns).
I'm not 100% sure if the loose/power saving MP fan clutch (6 cylinder van application if I remember correctly?)
will be adequate for you when spun at lower rpm. I think it will probably be fine but that is something
to keep an eye on.

Overall I'd say you are probably good to go.....but....."it depends"

"You have to test on your specific combination and...." You already know the rest

Imho, on the street, if you have enough motor, the power freed up by a pully swap will be too insignificant to notice. If you're not blowing away the tires, your motor is too small and the pulley won't change that.

If you have a voltmeter and a temp gauge in the car you are ready for testing. An ammeter might be even better for testing than a voltmeter but either should tell you what you need to know. I think it will work okay for your setup but you just need to keep an eye on the gauges. Now if you switched over to a Sniper setup you could just data log a few drives and you would know for sure what was going on with the underdrive pulley.

Brad - I tried an underdrive pulley on my old 63 4 door w the 512" and I did not have success with cooling. It had a stock BB radiator, flex fan w/shroud and it could not keep up. I switched back to stock pulley and cooling issues went away. Matter of fact, I realized the cooling issues when I was driving to the track. Part way there I pulled over to let it cool down and then limped it back home. I never did get to find out what it was worth on the timeslip. It might have worked with a better cooling system, who knows.

I think my stroked 440 runs a bit too warm due to an underdrive pulley. I have a 26" aluminum radiator, 160 deg. stat and a Milodon high vol pump. I swapped to an underdrive pulley mostly because I have an older Fluidampr that puts a stock pulley out further than the wp and alternator pulleys and the March underdrive is made to compensate for this dampener offset. The newer Fluidampr's have a recessed face to resolve this issue. At night with the headlights on at idle charging is marginal at best. I'm still looking for a stock diameter pulley with the offset that this March pulley has to improve cooling and charging at idle. Of course the pulley change was done at the same time that I swapped a mild 440 for a stroker so that is a huge variable. That's my experience, yours may vary.

I used to run the same setup on my 14:1 compression 446" wedge on the street w/ no trouble at all. Stock clutch fan, water pump, and alternator w/ a Moroso underdrive crank pulley.

Not sure where you're coming from with those comments. I haven't been worried about whether my car hooks on the street for 30+ years, since it's been that long since I street raced. My concern with using the car on public roads is purely whether the pulley change is likely to cause a charging and/or cooling problem.

BTW, on any decent track where I hook up, my car is traditionally very consistent, which is why it's been a good test bed to see what helps the car's ET & MPH. The crank pulley tests Dwayne Porter ran & posted years ago showed a Moroso underdrive pulley picked up about 10 HP at peak over an OEM pulley on a 500+ HP 383 he built. If I picked up 10 HP from a pulley swap on a 650+ HP 440, it would definitely show up on the ET slip.

We ran.run I still do March underdrives.My radical 440 runs under 190 all day with 22 inch rad.was stock now cheap Champion aluminum.My son ran the same with stock rad in his 62 300 and it also ran under 190 all day even in the 90s.never a problem charging or with temps.Ive run mine at least 20 years.My son een drove 40 miles to thetrack and back to race his 62.Rocky

I run the March underdrive set, no cooling issues, but at ~850 rpm idle I can sometimes overtax the charging system. even with the mini denso setup. But I've got elec fuel pump, elec fan, bottle heater...