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Fans that look like this:

enter image description here

Is it to somehow help the balance of the engine?

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It isn't something restricted to old vehicles; my Lumina has fans like this, though not as irregular as the picture shown in the question.

Lumina SS Cooling Fans

As far as I can recall, the chief reason cited for this by the manufacturer is noise reduction. You'll notice the additional weight on some of the blades to ensure that rotational balance is maintained despite the asymmetry.

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    It's true that a symmetrical blade would produce harmonic noise which would be more unpleasant than a more "white" noise produced by asymmetric blades. I believe another added benefit is maintenance/tool access because of the larger gap.
    – ghoppe
    Jan 7, 2016 at 16:32
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    This has also been done in recent Apple laptops for noise reduction - see here for a picture of the fan in a MacBook Pro.
    – Scot
    Jan 7, 2016 at 20:30
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    Many tail rotors on helicopters are like this as well...
    – Moo
    Jan 7, 2016 at 22:07
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    Might I add that many car tires also have uneven/staggered thread blocks to reduce road noise. Just look at the grooves on their shoulders and you'll see that the space between them isn't equal all around.
    – RaphaelP
    Sep 18, 2019 at 14:07
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The oddity is there to lessen fan noise. The uneven spaces creates air pulses that cancels out some of the noise, without lessening the ability of the fan to move air.

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It may be to allow a hand/tool to access a screw or bolt that would otherwise be inaccessible if there wasn't one opening between the fan blades large enough. Although I can't think of how that would apply to electric fans

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