I too have a 74 CB350F and am in the market for tires and am curiously watching to get some ideals also.
As with yours tires, mine are the factory tires, doctor had bike in his garage, only 2800 miles on it.
They still hold air, great tread, but has signs of cracking.
In fact, I wouldn't be surprised if they still have the factory air in them, as they haven't leaked a bit.