It's not what you want to hear....but $4200 for a stronger gearset, fitted, was an absolute bargain, no matter what the brand was.
I think you've been told a few times already in this thread....but I'll try again. Gearboxes have a few major parts that can break, and you have only changed the gears themselves. You are almost certainly running the original, factory selectors/forks/synchros/bearings etc. Any of those can break, particularly due to how they have been treated over the years (even before you owned the car). On the bright side they are much cheaper to replace the redoing the whole box, however it is still the labour that will sting (it takes a few hours to put box in and out).
On "whining", straight cut gears are stronger and whine. The noise is very loud and obvious. Helical gears are weaker and quiet, which is why everyone uses them. In pretty much every gearbox the forward gears are helical and reverse is straight cut. You can get quieter whining from worn bearings in the box but it is totally different to the straight cut noise. No "brand" of gear is significantly quieter or noisier.