8.5 should work with 275-295 tread width.
5.5 (you sure about that? I would think they'd be 6") would work with 205-225.

To me, a 50-series looks a bit too low-profile on these cars; 60- or 70-series tires just look most appropriate to me.

My deal is to fill the openings, with the nose slightly lower than the tail.

I just put 15x7 & 15x8 wheels with 235/60 & 275/60 tires on a '68 Road Runner, and to my eye it all works perfectly.