Another correction: Fleet option GMC trucks used the chevy dash. As for the "stabilizer"... Is this a cylinder that is mounted in a verticle fashion to both front wheels? If so, it is a gmc option called a road dampening shock. Basically the function is that the I beam axle will resonate at a certain freequency. The road dampeners are suppose to ressonate at the same freequency, but opposite of the axle cancelling out all road vibration.
The rest of the answers seem fairly accurate. A GMC is not an Apache pickup. The 100 on the grill makes it a 1/2 ton. Yes there were 1/2 ton long beds, yes there were 3/4 ton long beds. No, there weren't any 3/4 ton short beds.
The 55-59 gmc trucks were known as Blue Chip trucks, or 57 and later known as "Blue Chip Money Makers".
Engines are different between the chebby and gmc. If the GMC has a V8, it should be a 336, which is actually a debored 58 pontiac 370. If the gmc has a 6 cylinder, it should be a 270 cubic inch motor.
Chevy pickups came standard with an Eaton rear end that had a dropout style pumpkin. GMC trucks came standard with a spicer 45 rearend. The spicer rear was used for both chevy and gmc trucks with positraction.
Other differences are noticeable in paint colors, headliners, and seat fabrics on deluxe trucks.
Another subtle difference is the GMC Non glare dash. GMC painted the dashtops with a flat textured paint, while chevy painted it with a glossy paint.
Of course this is just a partial list of differences between 58 chevy and 58 gmc trucks.
