IF you are running the original radio, it will only support a single front speaker and a single rear speaker. (or two very small kick panel speakers)
If radio and speakers are new, you may be able to place small ones in the kick panels. Size is limited by the body frame in that area. If you are a purist, this was an option back in the day.
If you don't care about originality, you can chop up the door panels, rear side panels, back seat frame, or put some huge box speakers sitting in the back seat.
The 'stock' optional rear speaker for convertibles is about 99.5% useless with the top down. But that was actually an option back then.
I used to be indecisive. Now I'm not sure. I feel like I am diagonally parked in a parallel universe. 1968 400 convertible (Scarlet) 1976 T/A - 455 LE (No Burt) 1976 T/A New baby, starting full restoration. 1968 350 - 4 speed 'vert - 400 clone (the Beast!) 1968 350 convertible - Wife's car now- 400 clone (Aleutian Blue) (Blue Angel) 2008 Durango - DD 2008 GXP - New one from NH is AWESOME! 2017 Durango Citadel - Modern is nice! HEMI is amazing! 1998 Silverado Z71 - Father-daughter project 1968 400 coupe - R/A clone (Blue Pearl) (sold) 1967 326 convertible - Sold 1980 T/A SE Bandit - Sold