I have 7" on my 66
These are plenty wide, although the 8" will probably fit with no issues.
64/65 rear wheel openings are flat across the top--(picture 64 Impala)
66/67 openings are as big as the tires (if there is air in the shocks)
Personally, I think 7" is enough---particularly if the car is lowered a bit
Throwaway spares are fine for any car if the bolt pattern is right and yours is.
Just remember these are temporary both in use and in life. Keep them inflated per specifications at all times and check them periodically. After a few years they tend to deteriorate and will let you down.