Kei cars/trucks/vans are limited to the following requirements to be categorized as such per Japanese regulations:
Length − up to 11.2 ft Width − up to 4.9 ft Height − up to 6.6 ft Engine capacity − up to 660 ccs Power − up to 63 hp Capacity − up to 4 passenger seats Load-carrying capacity − should not exceed 771.6 pounds
1200 is technically possible. But it’s past designed brake, suspension, transmission capacity of any Kei truck. I’d urge you to use caution if ever doing it again. Wouldn’t go over 20 mph if you can even reach it at that weight.