The standard front axle rating was 4400 - 4500 lbs., there was an optional heavy duty axle available with a possible rating of 7000 lbs. Look up the GM Heritage Center and go to the Vehicle Information Kits, page down to the year and truck, then you can page through a lot of info on your particular model.

1957 Chevrolet 5700 LCF 283 SM420 2 speed rear, 1955 IH 300U T/A, 1978 Corvette 350 auto, 1978 Yamaha DT175, 1999 Harley Davidson Softail Fat Boy