Another thing is adding print grain, given the mission, would be excessive, since the best Blus today come from OCNs and therefore have relatively fine grain fields.
Maybe the way to go would be to try to replicate the grain field from a quality Blu of a film from the same era shot on similar stock.
Another option would be to analyze the grain patterns from the prints and then use a grain generator plugin, such as the one built into Resolve, to perfectly replicate it. It technically wouldn’t be “real” scanned grain, but the end result would likely be a closer approximation to the print than just hoping to find a grain sample that happens to be a perfect match.
Some purists might not like this idea, but this is a technique used in very high end restoration. IMO, the look of the end result is what matters most.
And FWIW, that’s how I got the grain to look perfect in my Fellowship of the Ring Extended color restoration.