In dash speaker installation on '66 GMC

[athan66]

I'm looking to mount a center speaker under the dash in the factor location. My main question is whether i need some sort of speaker mounting bracket since my truck orginally did not come with a radio.

I have purchased a reproduction radio and reproduction speaker that supposedly will fit in the factor mounting location, but I'm struggling to see how I attach the speaker.

Any help would be most appreciated!