Sounds like you've got a set of old speakers as well.
Early years had a single 4"×10" speaker in the center of the dash under the dash pad. Later trucks had one at either end.
6"x9" speakers will fit in the cab corners. Can do 5" down at the kick panels or underneath the dash facing down.
If you wanna get a lot better, can also do an amp to have a cleaner sound. An amp helps prevent you from cranking the radio. The higher the volume is set on the headunit, the lower the actual quality since you'll be much closer to its actual output limits.
So it's best to actually max around 75% of the volume and let an amp boost the audio.