The 1500-series was a vacuum booster, basically the same design as the average car. The 2500-series is a hydraulic booster, calibrated to assist much larger wheel cylinders and calipers than found in the 1500-series. I'm not 100% certain, but I think its a case of comparing apples vs. oranges.

OTO, this era Chevy truck has huge aftermarket/upgrade support. Perhaps such an upgrade has been done. I would recommend a web search.