This has been discussed so many times, where the 25 series trucks have a low registration GVWR of 10,000. Your 150 has about the same payload number, but a much lower RAWR. If you go over your 150 GVWR, you are soon to go over axle rating also. This is not the case on your brothers GM 2500. He will have plenty of RAWR/tire rating to handle the pin wt. If he scaled loaded truck, you need to look at scaled rear axle, say 3K for example. This would indicate he could handle about 3,200 more on rear axle, if rating is 6,200.
Jerry