It's a common problem on 318's and 360's. The intake has an access cover in the bottom. You must remove the intake to replace it. I would do a compresson test first on all cylinders, if they all pass, then demand that the intake be resealed.
Also inspect the plugs for burnt oil deposits. If you have a bad cylinder, put 2 tablespoon of oil and retest to eliminate any ring issues. If the compression test still fails, you have a cylinder head issue also.
If compression is good, replace the complete intake mainifold set, clean your throttle body and run some seafoam through the intake while the truck is running to help clear out all the carbon and oil deposits.
R&R the intake isn't bad, just time consuming for the first timer. You will not need to remove the distributor either.