Generally, once rust reaches that point on the cab you are farther ahead to replace the cab with a rust free version from out west or down south. depending on where you are located there a quite a few truck parts outlets that specialize in bringing rust free truck parts to the rust belt Midwest and East coast.
You can probably scab weld some support to anything solid enough to weld to, but don't consider it a long term fix as the surrounding sheetmetal tends to crack because it is now carrying the load differently than it was designed to....plus the rust keeps on growing.
Any 1/2, 3/4, or 1 ton truck hood from 1967-72 will fit your truck. Trim/chrome on the hood might be slightly different, but the hoods physically interchange.