Nice job! I've always found the 2x T5 exit signs pretty neat but they're not common here. There might be a date on the old ballasts so if you still have them it's worth it to check them.
I have looked at the stamping on the old ballast and did not see anything indication a date code. The back rest of the surfaces were smooth. Attached is a picture of the stamping.
The signs housing is made out of cast aluminum, and some of the corrosion is deep. I started scrubbing with simple green and scotchbrite pads and after getting nowhere I switched to barkeepers friend, steel wool, and a stainless steel brush. After an hour of scrubbing no more progress was made and I was tired. This is as good as it is going to get.
More pictures to come later.