I also wonder if the primer has anything to do with etching the painted surface for the butyl so adhere better?

The blocks in the kit will work...You're basically centering the glass in the opening...Do a dry run and flip the blocks and/or trim them if needed..FYI some factory repop kits have metal centering tabs but you'll end up with the same rust issues you want to get away from...
Also,You're linking the 3/8 kit everyone is recommending the 5/16 diameter kit..

And your guess is correct the primer promotes adhesion..You can buy it in a small can that has a applicator ball on the lid like a plastic PCV glue can has...Dip it in and run the channel with it...

Evan, Let's talk before you buy anything, I'll pass along my two cents worth on this topic.

I thought I'd recalled from previous threads on this that the primer wasn't necessary with the tape????

Happy to stand corrected on this......

As said above the primer is not a paint type of primer ( grey or red) it is a black liquid that promotes adhesion of the tape..

When I worked at a glass shop in the late 70's ( it is all different now) we used the 3/8" tape on all the W/S and 5/16 on the backglass. You can microwave the tape for a minute or so, to make it softer, when you install and you will get a better "set" as you push the glass down.. Or you can set it in the sun after you set the glass and come back later and push the WS further into the tape.. Basically this helps lower the glass height into the pinch weld making it easier to install the the chrome trim, while also giving you a better base ( using the 3/8" tape..) The 5/16" tape is a bit small for WS sets and does not "mush" into the nooks and crannies as well..

If you are going to come back over the top of the W/S with additional sealer, after setting it into the tape, try to find a more liquid type of sealer so it will "flow" into the area rather than just "sit" on top of the tape.. 3M had a product called "flow grade" that worked out best for an additional seal..
Additional sealant is not necessary if the pinch weld is in good shape and you get a good set into the tape.. Of course when I was doing it for a living we knew what we were doing also..

Remember if you break the WS it is yours.. If you pay for the install it is all on the installer.... Rick

good point on breaking the WS, worth considering. I remember one primer on one of the glass supplier sites was stating the primer was for sealing the pinchweld and covering up old rust or bare metal in the track but didn't mention anything about etching - maybe there's different kinds available.

I wondered Doug if you'd set your own, I was going to ask you anyways.