I have run extensive tests on all 6V red LEDs (and some yellow and white) which I could find regardless of dual or single element and regardless of polarity. Bottom line, they did not perform better than the existing incandescent bulbs largely because the directionality of light from the LEDs was not fully gathered and utilized by the glass lens in tail light of my 1950 Merc. That may not be a problem for you especially in older cars with limited tail light size.
If you decide to invest in 6v LEDs be certain of the polarity as most are made for negative ground. I purchased plastic sockets so I could try negative ground LEDs as well as positive ground. Note some rare types are non-polarized, i.e. will emit light regardless of voltage polarity.
I did decide that best light output was from a few single element 1156 LEDs in plastic socket and using main element for brake/turn and a dropping resistor for parking light function.
Note also that the two **** at the sides of bulbs which latch the bulb in the socket with the push-twist motion may be a problem. On single function bulbs they are at the same level on both sides. On dual element bulbs one is higher than the other so the bulb must only go in one way, i.e. so that brake/turn and parking don't get reversed. The problem I found is the old sockets in my car have a different level than the current bulbs, i.e. newer bulbs won't push/twicst and stay in place. It took some time before I happened on to the reason being a different height in some of my older parts. This is one reason which I used modern plastic sockets.
1156 1157
Best light output comes from flipflop.com but they are the most costly, $20 each. They come with a glass shield which must be removed for some applications. SuperbrightLED.com has a confusing catalog but once mastered their offering contains only a few 6v bulbs at good prices. If you get excited about a bulb I recommend only purchasing 1 to determine whether it would work. I spent $50 and a couple weeks on various bulbs in single quantities. And in the end I will stay with incandescents for now. I have nothing against LEDs but they are not the magic tail light which I'd hoped for.
I did evaluate the 1156 bulb which you linked to and I would recommend it as long as it phycically works in your tail light. Since my tail lights integrate brake/turn and parking, that bulb is once which I would use by adding a dropping resistor for the parking function.
Hope this helps
