The CCKW is rated by the manufacturer for 5,000.
Now there is a catch. The Specifications of the CCKW are for OFF ROAD/Cross Country. The War Dept in 1944 increased the allowable payload on HARD SURFACED road to a max payload is 10,000.
You will also need to increase tire pressure 1 PSI for each 1,000lbs over 5,000.
I have posted the info on my CCKW web site:
tm9-801.com • View topic - A CCKW can hold 10,000... you knew that, right?
This overload rating also applies to M35s. Off Road/Cross Country the max payload is 5,000, but on hard surfaced roads 10,000 payload.
Depending on the condition of your truck you may not want to attempt 10,000. But I have seen it.. a 353 loaded to 10,000 at Army Cars USA in WI. Its scary looking, drives like crap, but it will hold it quite well.
As far as 'expert' CCKW owners beware... I have spent the last 4 years correcting them.... make sure that every bit of advice is documented, not an old wives tale, or simply made up.