I have found the same rear housings on both trucks and cars. The speedometer drive gear determines which type driven gear to use.
An example would be: 55-59 truck with a 3:90 rear end would use a 19 tooth metal gear. It has a corresponding drive gear which is larger than used in a passenger car. If the same truck had a car transmission, it would probably use a 22 tooth plastic gear. The corresponding drive gear would be smaller than the truck drive gear to accommodate the larger plastic gear. Subsequently, if it has a car drive gear it won't turn a truck driven gear. Pick up a plastic gear - many on ebay for about $12 and see if it fits.
Fred
