Broken blower motor bolts 66 Valiant

Can any one tell me how to get the bolts/studs off or replaced for the heater box/blower motor? Looks like welded in?
66 Valiant non A/C car. one broke off repairing blower motor and replacing heater core.
I have other heater boxes and good bolts but the boxes are no good. I dont want to break or mess up the studs to get them out.
Thanks Andrew