I remember these measurements being awkward. The HKS tips slip OVER the 4.5" OD exit tubing of the exhaust, as you point out. From what I can find, there is no such thing as thinwall tubing with an OD of 4.625" that is in the US, which is what those tip tubes are, iirc. The Afterburners are designed to butt up to the 4.5" exhaust exit, with the tabs sliding inside the exhaust section where they will pick up the mounting points.
I was definitely looking at different tips though, as these shown here are a couple of inches shorter, and the Afterburners, while mounting to that, will extend out that difference in length. At a glance, it appears these will extend the entire length of the fins. Let me look at something.
Thanks for the info! This is good data. There is no substitute for getting a hold of a local HKS car, but this may suffice. We have the technology.
Here are some updates of the updated design progress.
Send me a pic of the HKS exhaust on a car that is further away from the vehicle, please.