The 6973s can be made to work in amps that came with 6CZ5's, so 280 owners are in luck too. The pin-outs are the same except pin 8 is not used on the 6CZ5.
It is important not to go in the opposite direction (sub 6CZ5s where 6973 tubes should go), the 6CZ5's max plate voltage is 350 and the 6973's is 450.
Servicing the power tubes in a 60 year old amp should be done with care. Consider having a professional amp technician give your amp a health check when changing power tubes, especially when subsituting type of one power tube for another.