You don’t have 48 deg of total. Any number of degrees of timing added with vacuum advance (either manifold or ported) goes away when manifold vacuum drops (IE you’re on the throttle). It is completely reasonable to have as much as 48 or 50 degrees at cruise when vacuum is high. And no you won’t see detonation there.
My initial timing (without VA) is set to 32 degrees. Mechanical advance is limited to 16 degrees and is all in by 3500 rpm (silver springs on a billet Pertronix). Anytime rpm is above 3500 my total timing is 48.
At a 2000 rpm cruise with 15-20 in vacuum, I may as much as 32 initial + 16 VA + 10 mechanical = 58 with no surging or pinging.
I am going to check the TDC mark on the balancer again and make sure it is accurate, but I think it is. I have an MSD ignition, so timing light could be off, but I have checked timing with both an old Craftsman induction light and a new Innova adjustable light. They both read the same.