Hot Rod TV has a short clip on the early history of belly tank racers on the salt flats.
An extract from a history of belly tank racers. This disagrees with the video on one small detail. It says the first tanks used were from Mustangs, before moving onto Lightnings, whilst the video says they went straight to Lightnings.
The Monster Garage TV show made a belly tank racer from an F4 Phantom tank.
It should be possible to find a belly tank from the spares box if you’ve built enough Second World War era fighters, though it will probably be in a non-traditional scale for car modelling. A 1:48th speedster could be tended to by remodelled air crew, but a 1:72nd one would be tiny- though I guess that makes it more of a challenge.
Update There are belly tank racer models available. This online magazine (pdf) has a feature on one modeller’s attempts and the three resulting models.
For more inspiration-