Based on your wording, you seem to want to stay factory original.
Based on the AMA specs and sales manual, all '67 Firebird models had the 14x6 wheels and E-70 14 tires (with one optional size, a 185R-14 rayon radial)
The '68s also used a 14x6 wheel but used both the E-70 x 14 and F-70 x 14 (and an optional rayon radial size) based on the model and options.
(Personally, I focus on originality too. However, if you decide to stray from factory original, there is a lot of non-original tire size discussions that can be searched in this forum.)
