Rear Package tray Question (66 coupe)

yep, put in underneath the metal pieces. Also, you might want to get some vinyl, foam, and spray adhesive to make kind of a cover for the fiberboard. I always thought it looked tacky just having painted fiberboard back there, not to mention that it rattles because of the speakers. Some like carpet(that short utility carpet stuff), some like vinyl, evidently some like fiberboard. Just throwing out an idea.
 
I just finished doing this a few weekends ago on my 66. I bought the fiberboard, and went to the craft store and bought some quilt padding and some black vinyl for around $10 and padded the fiberboard. Then put my 6x9's in. Came out nice and put the lip of the sheet metal on top of the fiberboard.