First welcome to the site. Hope you find the support and resources needed for your restoration related needs. Thanks for including the plant and production period in your first post!!
Your answer depends on what part of the underdash area your looking to redo. It also depended on the effort of individual painters put into the task and well as allot of those areas had restricted access so they would receive no paint leading to surface rust on some/many cars depending on where the car was delivered and how stored.
For many answer a good place to start for many things would be the Unrestored Picture section of the site but since your question is best answered by looking at disassembled examples which typically don't get posted there. You may not be aware of this section so now that you have joined take some time to familiarize yourself with at least the area of the site. Search is another valuable resource as Jim mentioned above. Like all search engines you need to play with combinations of words at time to sift through the possibilities
To you specific question there would be no grey primer typically since it was only applied to the exterior of the car prior to body color and over the red oxide.
Some red oxide from painting the floors but depended on how much effort the painter put in and how far forward and up the firewall the guy went that car.
Possibly some interior color since the dash was applied first over the red oxide but before the body color when the dash and A pillars were masked off. Paint could enter through the glove box, gauge and radio openings but that would still be limited.
Finally some body color around the outer edges from painting the forward door jamb but the bottom of the dash would limit the extent upward at that point and from the vast amount of overspray you would get some paint typically lower on the firewall area.
These practices would basically apply to all Mustangs built at San Jose during 65 production
Difficult to find or collect pictures of this area. So have two examples showing the range of possible results of how things could have turned out.
In the first one the painter extended the red oxide far under the dash and upward. Also looking through the openings in the firewall pad gives us some insight into that surface behind the pad. The challenge here is that the exterior and interior were pretty much the same color so we don't know which one was applied to the greenish finish we see. Important to figure out the difference if your interior and exterior are not basically the same color
Bare steel with very very light overspray
Red oxide above - green directly behind and through the master cylinder brace area
Second example where the painter applying red oxide did not extend the paint under the dash much nor upward. Instead we see areas that have no paint and others where we see the interior paint color introduced through the glove box opening or from painting the lower edge of the dash pointing the gun upward producing the results we see
Had one additional example from 65 SJ but it is an odd one so didn't want to confuse anyone. Appears to be a factory repaint
