I say they're all good choices, with pros and cons to each. But no cons that would really be a deal-breaker for any of them.
ZF and 4500 are both large truck transmissions that require (I think) a 2" body lift or tunnel modifications. Both require adapters to the transfer case, but the ZF can just bolt right up (relatively speaking) to the bellhousing of the 302, while, if I remember, the 4500 requires an adapter.
Both have lower 1st gears than stock or the 3550.
Both shift better than an NP435, but are still somewhat truck-like in their feel. Overdrive is decent on both.
Both would likely require driveshaft modifications to to the longer overall length.
Only the ZF was available in a Ford, so for the purist would be the choice in that regard.
Pricewise though, I'm not sure about the initial acquisition (though the 4500 seems to be more common?), but you can save a few bucks by not needing an adapter for the ZF.
The 3550 on the other hand, is smaller and lighter duty than the other two, with a taller 1st gear. Priced new, they're not far from a 4500. Priced used though, around here you can often find a 4500 cheaper than a 3550.
The size allows you to fit them with no driveshaft mods (roughly 1/2" different from stock, if that). They supposedly shift a bit smoother, but I'd say they're close to the same in shift feel to the other two.
I"m probably missing something, but all in all, they're all three good choices, with lots of proponents here in the forums. I'm sure you'll get lots of personal experience stories here.
Me? I'm going 3550, mainly because of the size and weight.
Paul