Probably been answered before but is red oxide a good choice to use as final coat for the underbody or is it pourous?
Not as long as you use a product like Ford originally did - an epoxy primer sealer like the DP - line of PPg/Ditzler
Otherwise what paint code is close in colour that can be used as a topcoat?
My car is a 67 fastback gt390 auto San Jose June built.
You can use the product mentioned above (mentioned in at least a dozen earlier threads) as a base coat for body panels but I would check with the painter since they each have what they like to work with and what will not react with the top coats.
As for undercarriage nothing IMHO is going to be right - right out of the can. Remember that the tint on the coating from the firewall forward was often different to see when compared to the finish from the firewall rearward.
Best to examine your original floor pans and frame rails for original examples - best source. Without that your guessing and looking at what other cars from June/San Jose have
Examples I have of that period and plant are very similar to mid to late 66 - a muddy red look
