Double stick tape and spray adhesive are both EXTREMELY bad for prints. They'll cause the prints to deteriorate far earlier than 50 yrs. For archival mounting your easiest choice is LineCo plastic mounting corners. They're made of archival plastic and no adhesive of any kind touches the print (The sticky part, which is also archival, sticks only to the backing board). They're fairly cheap, easy to use, and the print slides out of them easily if you need to un-mount.