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Use a *proportional* trailer brake controller, ensure your suspension height stays at a stable range ( install helpers if necessary), use a weight distributing hitch if at all possible, and assuming your brakes are of good quality semi-metallic or carbon-metallic, calipers working smoothly, rotors in good condition, all aspects of the rear brakes working correctly, wheel bearings nice and tight, hitch of the proper capacity and design, etc, it shouldn't complain much. My 76 k20 is a 6800 lb gvwr flavor with 4.1x gears, sm465, and np205 but has since been upgraded with rear disc brakes, hydraulic trailer brake controller, front PFC carbon metallic pads, Wagner rear semi metallic pads, new calipers all the way around, larger bore master cylinder, 4 row factory radiator from a 1980 c10 diesel, wix 51794 larger capacity oil filter, 1987 TBI swap, probably other stuff I'm leaving out. I wouldn't hesitate to drag 10-12k behind it if the hitch was up to the task. It seems to tow about as well as my 99 c3500hd does, but the 99 doesn't have trailer brakes, and in its utility body/former bucket truck dress, boxes partially full, bed empty, easily weighs double what my 76 does empty. The 99 runs a much higher gear, something in the low 3's, and its a 4l80e so its vortec 454 is MUCH happier at modern highway speeds than my 76 with a TBI 350 is. Assuming the mechanical basics are up to task, truck and trailer brakes are good, keep your speed around 55-60 mph or less and drive happy.