I have a 72 Challenger with a big block.
When I make a pass, the fan belt comes off. The belt is a "V" belt and drives the water pump and alternator.
I have checked pulley alignment and I can't find anything off. I have tried different width belts. Right now I have a pretty skinny belt on and seems to stay on but it is not right.
I really do not want to go with a serpentine setup.
Any ideas?

Is your alternator bracket(s) stock for the big block you are running? Is the length of the belt longer than necessary causing a longer span from crank pulley to alternator?
Almost without a doubt the belt is whipping at speed in this span and eventually loses contact with the alternator and comes off.

Is it simply too loose? Is the alternator adjustment bolt slipping?

I had(have) the same problem with my then 318 powered Challenger.
I used to autocross a few times a year with the car, it was common to lose the fan belt during a run.
Someone suggested removing the fan and fan clutch before a run.
It worked, belt stayed on! If you're still running a stock style clutch and metal fan I'd try a run with fan removed.
I'd drive to the event and remove the fan, wasn't too hard to do, and replace for the ride home.
I don't know if there's a lighter fan and clutch I can try. Or if the fix is going to be adding an electric fan.

The alternator is in the stock location but I think you hit on what my thought was.
The length of the belt between the crank pulley and the alternator. That is the only thing I can come up with.
I have tried to find a deep pulley for the alternator to see if that helps but haven't had any luck finding one.
I am looking into a tensioner on Mancini Racing but I'm not sure if it will fix the problem.
The pic I posted is not a good shot but you can see the location of the alternator.

I was running a fan but have since converted to electric. I thought the centrifugal force of the fan (non clutch) was helping throw the belt.
The belt stays on but it is the skinny one I mentioned.

How are you checking your pulley alignment? Are you using a straight edge, a lazer pointer or a wooden dowel? It's not as easy to do correctly as most people think, and close enough often isn't. What thickness belt are you using? Does it fill the entire v-groove of the pulley? Does it stick up beyond the groove?

Look closely at each pulley where the belt begins to contact the pulley. There are two places to check on each pulley. Does the belt touch one side of the pulley wall more than the other at these 6 locations?

More pictures of these locations would be helpful.

I used a straight edge to align the 3 pulleys. I had to make minor adjustments to the alternator to get it aligned.
I have used a belt that fit correctly in the pulleys which is fine around town but not at the strip. I used a belt that stuck up out of the pulley which I know is not correct. That one didn't work.
The only one that stays on at the strip is the skinny one. It sits way down in the pulleys but isn't really right either.
Sorry, I don't have any better pics on my computer to post and I don't have the belt dimensions available. I bought so many different sizes I don't remember which is which.

Easy fix put a dayco top cog belt on it and that will fix it. I couldn't keep a belt on my hemi and then switched to a dayco and haren't tossed a belt in over 10 years. I use a 15470 belt.

Mike

How much wrap around the alternator? Is the belt touching between 1-9 (clock) or 12-6........
Longer belt and swinging the alternator as high as possible will give more contact area.

They cured that problem on the Max Wedge by going to deeper groove pulleys. IOW, a 3/8" belt in pulleys designed for a 7/16" belt. (truck?)

Had the same problem with a high strung 360 in my car. It once threw the belt off and over the fan without catching the fan( do NOT ask how! Still miffed) i see you have a double groove pulley on the alt. I had a/c pulleys on mine as well, no a/c, so I thought I'd just run 2 belts. It kind of worked, but I did remove my fan and shroud asm and went electric. Much better.

You might try loosening up the tension a bit. Sounds crazy, but it works.

I had the same problem with our 69 Dart 383 GTS that has a 451 stroker in it.
It threw every standard type V-Belt. I switched to a DAYCO top cog V-Belt and haven't thrown one yet.
The Dayco Top Cog Belt sits lower (farther) into the pulley grooves when installed.

Pato....here’s another idea. If you take your timing light and point it at the belt, while reving the engine, you will get an idea of what the belt is doing under various conditions. It helps determine what is working (like tight vs loose) and what is not.

Had the issue on my small block. I would tighten belt REALLY tight and it would live for a bit. Not really fair, I shift it at 6400 rpm. What really stopped it was a March under drive crank pulley. Remember one of the first gauges you check after the stripe is the alt gauge. Not charging the belt is gone. You catch it much quicker that way than seeing it start to get hot.

I had this problem with a fast revving small block. Checked/adjusted pulley alignment multiple times, tried a boat load of different belts. Even tried a larger alternator pulley, nothing really worked. I finally put a reduction pulley (smaller diameter) on the crank and went back to the stock alternator pulley and the problem was solved.

Post the brand and number of belt you now have on the car.

Stock location but it looks a little high, do you have a picture of your alternator bracket? It could be the angle, but make sure you don’t have an AC lower bracket, this makes the belt needed much longer.

What do you have for a fan?

Put a straight edge on the alternator pulley. My bet is that its centerline is off in at least one plane. Or the pulley centerlines are forward or back from one another. The straightedge should stay perfectly square to both the crank and and water pump when placed on the alternator pulley. If the straightedge doesnt touch both faces of all three pulleys you have an issue. Do the same with the crank and water pump. I've had combos throw belts. Every time it's been one of 2 issues. Alignment or bracket flex. Currently have 3 cars. All have modified brackets, pulleys, etc. None of them ever through a belt. All run to at least 7000.
Doug

and will add
the longer the distance is between the pulleys and the less contact around the pulley the more prone they will be to throwing the belt.
Some of the c-2, c-3 corvettes used an adjustable belt guard that would not allow the belt to raise up enough to come out of it's groove. This was placed on the adjusting arm between the water pump near the alternator. NOTE: the alternator is the smallest pulley with the least amount of contact. This was on cars that had L (drivers side) mounted alternators.
Another trick from years ago was to place a long 5/16 or 3/8 bolt just above the belt to limit its upward movement thereby kepping the belt in the pulleys. Again this was done between the alternator and water pump.

Interesting that several have commented to Dayco top cog belts have resole their problem. I do know there are some slight variations on the belt V angles being made by different manufacturers. That may be the root cause