[bigblockv6]

The 3/4 truck would come with either 4.10 or 4.56 ratio,if it hasn't been removed over the years there will be a tag attached to the rear diff cover identifying the ratio. As far as what ratio was optional I don't think that really was the case, there were just two ratios available for that tonnage model. Unless the truck was a special order and the customer specified their choice of available ratio you got what ever ratio the dealer ordered. I know when my father ordered out 68 K2500 he never specified what gear ratio he wanted and the truck came with 4.56 ratio which he felt was just too low for his needs comparing it to his 62 that had 3.54 gears that seemed more than adequate with all the torque of the V6. Pete Chronis Northern Cal.