If you don't want any headaches go with Precision or Steele. Besides fitment you want the gasket to have a soft rubbery feel to it in order to make it easier to install. If the gasket is hard it will make the installation hard.
When you are speaking of "spline" .... if it was a
lock strip that is used to tighten the gasket to the glass .... that is a 67 gasket. Mopar used that system on the 67 only then switched to the
self locking on 68 and 69.
All and all if you have the stainless trim you have the 68 .... you will need the self locking.
I have a 67 cuda and just converted mine over to the 68. I purchased the trim from a member and bought the 68 gaskets. Mine initially had the lock strip but I was missing the corner pieces and really didn't like the lock strip look.
It's been about 8 yrs ago .... and right now I can't remember exactly which company I purchased the gaskets. I do remember that they was expensive but I didn't have any headaches in installation. I'm thinking it was Steele.
After getting back started on my fastback cuda this year I noticed that the rear glass was leaking at the rear corners. I fixed it buy using a non harding sealant that is used on RVs windshields and it worked like a charm. This stuff is better than the traditional bed and glaze. Less messy as well.
I've worked for Safelite and a auto glass shop so if you need any advice I might can help you. Just PM me if you like.