And I'm not sure if those are retaining springs as I can't even slide the black plastic far enough to know. What I'm going to do next time I get a chance is take a measurement of the stock exhaust from the rear tip to where they attach to the stock header pipe. That way I have a base idea of length of the pipe when attached. This should help me in determining how far before I need to try and pull on those springs..My Mistrals didn't use this arrangement...
These are retaining springs (inside the plastic covering), right? I've seen a number of competition pipes that use this method, usually with no other clamping method. Anyway, install the slipons as far up the header as they will comfortably go, hook one end of the retainer, and use vice-grips to pull hard enough on the other end to bring it to the other hole. Or maybe use a rod of some sort to lever the spring's free end into position. Have you tried emailing Agostini (or maybe Moto International, AF1 Racing, etc.) for a copy of the mounting instructions??
This shouldn't be an insurmountable obstacle. Good luck...