The 48-50 trucks had 3 ways to mount a radio. The in dash one was like a 49 Merc except it was beige not chrome like the Merc and mounted differently under the dash. Many have adapted them to fit, as they are a lot cheaper than an original truck radio.
There was a kit to adapt the 49-50 car radio into that same opening. It bolted to the dash with 4 bolts. There was also a kit to mount it below the dash.
The 51 dash was completely different and the radio was in the lower part of the dash. Those, you could just use a car radio in the opening. There were 2 versions of the car radio also.