Holy cow. Well, you shouldn't need to "pound" UCA's in with a rubber mallet. Or any kind of mallet really. If they don't fit, you need to look at the UCA mounts and see what's going on. The mounts can get bent a little, usually what happens is they get drawn in when the UCA bolts are tightened so sometimes they may need to be spread apart slightly for the UCA to fit in properly. I wouldn't call knocking off a bump stop tab with a mallet "breaking" a set of QA1 UCA's either. Maybe you got one that was a little short on weld, but that's not normal use either. It's just a little landing plate, and the load it's supposed to see pushes it against the UCA, so it doesn't need to be all that strong. QA1 makes great stuff, they're one of the better companies for aftermarket suspension parts.
As for grinding clearance, it's a pretty common thing to need to do with any tubular UCA. It doesn't hurt anything, the mounts just weren't designed for a tubular arm and they're not even all the same from the factory anyway, some are more restrictive than others. Something like this usually takes care of it, you want to take off as little as possible but anything up to the mark indicated won't really weaken anything.
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The stock UCA's are pretty flexible, so, they have to be completely supported in the area around the bushing when you're using a press. More than a few people have found this out the hard way by bending the UCA.
The UCA's are not drum or disk brake specific, as hemi71x pointed out. In 1973 they were upgraded to a larger ball joint (the same ones used earlier on larger B/E bodies), but there isn't a "drum brake" UCA. 1967-1972 used small ball joints, 1973+ used large ball joints. But otherwise the UCA's were the same.
The stock drum brakes may not be all that safe themselves, especially in modern traffic. Especially if they're 9" drums and you're used to driving modern cars.
Stock UCA's with stock UCA bushings won't give you enough positive caster for a proper alignment if you're using radial tires. Moog k7103 offset bushings can be used to get more positive caster, although in most cases even with the offset bushings the best you can do is about +3*, which is about the minimum you want with radial tires. That's why so many people go to tubular UCA's, because most of them have additional caster built in. Food for thought.
And, sorry, but it sounds to me like your disaster has been mostly self-made. If you're not very experienced working on older cars, you may want to slow down and ask someone or consult the factory manual before you break out the mallets and presses. Nothing wrong with doing it yourself and learning, that's awesome. But it's pretty easy to screw stuff up if you don't use the right technique or start beating on stuff with mallets. Most stuff can be replaced with aftermarket parts, but there are still some parts that you can screw up that aren't easily obtained.
You can download a 1968 shop manual here...
http://www.mymopar.com/downloads/servicemanuals/1968_Plymouth_Service_Manual.zip
