To answer the question on moving the gas tank to under the truck bed is an easy one, use the 1947/55 Suburban/Panel gas tank. It will fit under the bed easily and mounting hardware is available too.
If you are getting exhaust fumes in your cab, you can have a major and deadly problems. Carbon Monoxide is in those fumes and can overcome you if you are driving long distances with the windows rolled up tight. It may be more than a manifold exhaust leak, it could be your whole system needs to be replaced.
Another possibility is that it could be coming from your blow-by tube and your engine may need to be rebuilt. Put a CO2 alarm in your cab while you are driving and see what it does, my guess is that it will be blaring and loud. Just my 2 cents.......g
