The FJ40's were built like a tank and weren't prone to being rust buckets. Now days they are not good daily drivers because they can't keep up with modern traffic. The Toyota F-Series (GMC/Chevy licensed knock-off) I6 engine was a tough and reliable engine with good low-end torque, but it was a low-revving/low-horsepower mill, and the 4.11 gear gave them a top speed of "maybe" 65, not to mention the fact that crude 1970's emission controls applied to a 50's era engine design really hurt its performance and drivability. Later versions had a 3.70 gear, an improved head design, EFI, and a 5-speed transmission which significantly improved their performance and drivability.
Since the SBC V/8 engine was a largely a simple and inexpensive bolt-in proposition, and the Landcruiser's drive train was capable of handling the additional power, it isn't hard to understand why so many of them were upgraded with them. If I had the choice between one that has been upgraded with a V/6 or a V/8, I would take the V/8.