I can only tell you what was suggested by Silver Bullet Pipes about my K/B 3.5 SS Air Cooled Outboard.

With the Silver Bullet Header and the Late Style Pipe, the header pipe should be cut approximately 1/2" from the end point of the radius.

I had to call and speak to him about it, because there's really nothing as far as instructions on tuning, and this was the first time with me working with outboard pipes.

Sorry, I can't be more help to you, but what I understand there's a common length to cut the header, and the rest of the tuning is left up to the length of the coupler.

My plan is pre-marking the cutoff points at 3/4", and removing 1/16" at a time with a file and running it after each cut. Hopefully I can tell where it starts to drop off and have enough smarts to stop cutting. This will keep the gap between the header end and the pipe to a minimum, which will be easier to adjust when it comes time to replace a coupler.

Either way, for me it's going to be a lot of guesswork and testing. It sure would be nice if the header pipe fit into the pipe opening like a trombone slide, and just use a coupler to lock it in place, but it's the same size.

I hope someone else that has more knowlege than me will write.