Consult your sales rep
Consult your sales rep
I am assuming you haven't called your tech or Sales rep for help on this but the KM C451 should only run 300 GSM out of the bypass tray. You will also need to set that tray to thick 3 or 300 GSM, at the device and in your print driver, as it needs to adjust for that thickness.
In all reality you should be able to call your sales rep and or tech to help you with this. Even if you don't have a service contract. If you bought this from them they should train you how to use it.
In regard to the registration...this is an office machine and while it is capable of a lot it still will have issues registering from front to back consistently, especially on heavier/specialty stocks. A good tech could keep it somewhat in line however if you would like better registration and throughput capabilities the KM C5501/6501 would be a better fit.
The KM C451 is great in office and even very light production environments however I would not sell the C451 to anyone who plans on running stickers all day, blue back or not, or who requires consistent front to back registration under 1 mill. Those types of requests take me right to the C5501/6501. Just my 2 cents.