'68 Camaro Rear Spoiler Repair

[Mkelcy]

The studs that hold the spoiler to the trunk lid have pulled out. I drilled the holes out to about 3/8ths, and want to fill them with a material that will bond to the plastic(?) of the spoiler, but could also be drilled and tapped for replacement studs. I was considering this J-B Weld from McMaster Carr: http://www.mcmaster.com/#catalog/118/3404/=iz6bht or this product from Aircraft Spruce: http://www.aircraftspruce.com/catalog/cmpages/fixit.php

Any thoughts? Also, would I be better off with 1/4-20 or 1/4-28 studs. My thought was more, finer engagement points, but the 1/4-20 may be better with deeper threads.