I used the RetroSound 3.5". It's a fair sounding speaker. The best would problablty be the Infinity reference speakers, but they are costly. Then, no 3.5 " speaker is going to provide bass like a 6 x 9 speaker. It basically will supply treble and midrange frequencies. There's just not much room in the opening provided in the dash of a 71 B-body. You might be able to fit ( with mods to the speaker) a 4 x 10 with the dual voice coils.
Ron