I have seen that panel adhesive fail on body shop work. Best bet, put the plate behind the B pillar and forget about it! Seat belts are an excellent idea. Have them in my 47. Riding along without them is fine till you have that accident! That is why they call it an accident, you never know when and there is no time to put that belt on. I have seen what ejected from a vehicle does to a body. No thanks.
(Not that hard of a job)