A Ford 9 inch is not big enough for a real truck. Plus the brakes that come with it won't be up to the job.
Are you making other changes to the frame and suspension?
You can cut down the truck so it won't ever carry a load, but a 4400 came long enough for a 12 foot bed and designed to stop a total of 14,000 pounds. Empty with no bed this truck weighs about 4,100 pounds.
Changing to passenger car stuff without making changes that limit how much you can load on the truck creates a dangerous situation.
While the 9 inch was used under stuff that might weigh empty about the same, it was never used in anything designed to gross what a 4400 can carry.
What are you planning to do with this truck?
