I have seen a factory package of the two (one short, one longer) dash pad bars at Larry Stock's shop in Nevada. The embedded studs remind me of the same arrangement that holds the original vinyl down on the firewall bubble between the seats- don't know what thread but tiny-metric. The hex nuts will be above the glove box so that must come out for installation,
Consider if your vinyl is bulging away from the fiberglas due to shrinkage, that installing some kind of bolt-down bar to draw it back may overstress the vinyl to the point of tearing it. Heating it beforehand may help. I personally do not need the minute amount of flat area I would gain, enough to risk destroying the OEM vinyl.