I was gonna make this post yesterday too, I'm pretty sure it's the 80mm, but I couldn't find the details, I even looked at the JLT intake for tube size. I was wondering if gt350 throttlebody would be a $199 upgrade path and if the gt350 JLT would fit. I'll call JLT tomorrow.
Okay so to answer your question, yes its 80mm and you can ask JLT to put the gt350 coupler in the kit if you bolt on a gt350 throttlebody, however. the throttle eletronics are a lot different per Jon from Lund, so tuning them requires R&D.