I am a little confused by your post. Are you trying to waterproof the whole truck or just ensure it will continue to run while doing water crossings/fording?
If it is the first, I do not think this is cost effective since these truck were never made to be watertight.
Here is my .02:
If you are just trying to ensure it stays running then IMO you need to do identify two things first.
1.
Determine the max depth for water. This will usually be lower than you would expect or hope. Looking at this realistically you would think this would be the mid line of the window opening. This may not be the case as the truck will want to "float" after being under a certain degree of water not to mention the driver will need to breath.
Depending on the weight of the truck and how you rig your extended breathers you will have to determine this height. If you do water crossings often, I would put a discrete be high vis mark on the inside of the cab to give reference. Did this in the old days on HMMWV with fording kits.
2.
How are you going to ensure components continue to work. You mentioned extending the breathers which is good but if you have water over the hood, where are you extending to? A snorkel should be in your future and if so, you could run the extensions up to the hat. This will place them far above what you will be able handle.
There are other things you should consider as well, one you have mentioned. You could dielectric grease on the battery terminals but think about the other electrical junctions that are essential. How are you sealing your intake/gas tank/oil/PS/transmission? Mechanical fan?
Doing water crossings are one thing but fully submerging a vehicle is never a good long term plan for many components to stay reliable.
Interested, please keep us posted on what you end up doing.
