If you are using LEDs in your stock tails, you will the SRCK bulb for the middle socket as a regular LED does not work. Trust me, I've done it before, I've used the Zevos.
Don't make the same mistake I did. Plus, they point backwards, which makes little sense to me. I think the tails are more suited for incandescents, though.
In all honesty, I wouldn't even touch your stock tails, just get some LED tails, they're like $100, well worth the investment, couldn't be happier!
https://www.amazon.com/SPPC-Clear-L...-4&keywords=88-98+chevy+truck+led+tail+lights
With the Sylvania's in the side markers, I've tried them and they didn't work, you need to get non-polarity bulbs as the Sylvania's are polarity sensitive (they are great!):
https://www.amazon.com/Alla-Lightin...keywords=non-polarity+amber+194+alla+lighting
Now, the Sylvania LEDs are hella awesome for the interior! Instrument cluster, courtesy lighting, etc. I love them, and trust them.
It's your choice though, not saying what you should get, it's your money and truck.