For anyone in a similar situation, I can confirm that the Supra lower control arms (LCA's) will fit on a Lexus SC-300.
Me: 1999 Lexus SC-300 (136k miles) with bad ball-joints.
Relevant Parts Info:
96-00 Lexus SC-300 LCA's =
RF - 48068-29215
LF - 48069-29215
97-98 Supra LCA's =
RF - 48068-14090
LF - 48069-14090
93-96 Supra LCA's =
RF - 48068-14080
LF - 48069-14080
I bought the 93-96 Supra LCA's and had them installed without any fitment issues, and afterwards had an alignment done without any issues either. The good news is that everything fits as it should with a cost savings of over $1,000.00. Now for the bad news. There are only two negative side-effects I've noticed after 1-week of driving with the new LCA's. They are:
1. The stiffness of the new LCA's has changed how the car rides. Specifically, when going over bumps in the road, I can feel way more vibration than before. I can hear and feel every reflector I drive over when I'm changing lanes and it only gets worse with pot-holes. However, it's not clear whether this would have also been the case with new Lexus LCA's also. The new rubber on the Supra bushings and ball-joints is said to be stiffer than that of a Lexus, so I would imagine that it would at least have been slightly smoother with Lexus LCA's. Still worth the cost savings to me however.
2. The ride height of the front end appears to be slightly higher. Again, it's not clear whether this would also have been the case with new Lexus LCA's since the rubber on the old ball-joints had deteriorated and may have compressed under the the weight of the front end. The difference in height that I'm seeing now may actually be the correct ride height that the vehicle would have had when it was new. Still it looks odd to me since I'm not used to it.
Other than those two things, I'm satisfied with the Supra LCA's since it has definitely improved the handling of the vehicle and saved a significant amount of money in the process. Pics below:
Side-by-side comparison. LEFT LCA = Stock Lexus LCA (I'm told it's aluminum, hence the ridiculous price) RIGHT LCA = 93-96 Supra LCA (Supposedly cast-iron and heavier than stock LCA's)
Another side-by-side. The only noticeable differences are that the tab sticking out near the rear bushing, and the small lip surrounding the Lexus LCA rear bushing, are both missing on the Supra LCA. Not sure what they were for, but it's doubtful they make much of a difference.
Here you can see the difference in the worn-out ball joint vs. the new joint on the Supra LCA. This is probably why the vehicle has a slightly higher height on the front end now.
Finally, this is the Supra LCA installed on my Lexus. It does indeed fit.