The silicone connector is just to keep gasses from leaking. You can slide it maybe 1/4", but it's not for tuning. For one thing it will not last very long if too much is exposed to the exhaust, and it will sag and pulse if there is too much exposed in between the pipe and header.

Use tubing cutters

If you just buy a pipe and header, and put a 3" piece of silicone between them, you are gonna be way long..unless you might be running the absolute largest prop your engine can turn.

So knowing this..the header folks put extra tubing on the header..so you can cut it off until you get it set for any prop smaller than the absolute largest prop your engine can turn.

Don't sweat cutting the header..it's therapeutic to make a cut and see the engine perform better. It's the way it's done...