Landlocked...to answer you question requires a number of considerations

1. Our typical model boilers when operating at say 3 Bar raise steam to approx 150 degrees C
2. 450 degrees F = 232 degrees C....so one may think that sounds acceptable, however we must consider factors of safety...the example here provides a factor of safety of approx 1.6:1 ...where as the requirement may be in the order of 4:1 or a silver soldering melting point of approx 600 degrees C
3. Certainly in UK & OZ, the hydrastatic first test of any model boiler is
twice the rated working pressure
3. I see you are from USofA...I would be inclinded to ask a local model boiler inspector or certifier ...if he would certify a boiler constructed with 232 degrees C solder...my gut feeling is

4.
www.cupalloys.co.uk provide a worldwide service in the supply of silver solder wires & flux's & even have a Modeler Section in their WEB page...most of their silver solders are in the 600>700 degrees C melting range & hence offer an acceptable factor of safety
Keep us posted with your thoughts............Derek