Xenon,
With all the info you have received on your 400 TBI engine, You should be able to make an somewhat decision. I don't know how many dollars you have to spend on this project but to get gas mileage out of a 6500# vehicle that has the areo package of a brick, is going to be very difficult. Being a 400 is a good start in that vehicle, most chev camshafts are ground for a 350 application in the catalogs, unless otherwise stated, so in a 400 the cam will seem to be smaller due to the larger displacement of the engine and torque output. Therefore you can use a Slightly larger cam duration to achieve the desired results. For instance, a 204 duration camshaft works very well in a 350 application, in a 400 you can use a 209-212 and get better results and approx same power band increased toque, hence the increase in CID, and Stroke of the engine.
You have stated what heads you have, example- casting number. If you want to change them or not, again goes back to dollars. Newer heads work better than 80's open chamber emission heads by far. As was stated earlier install a set of vortecs and go. However that will involve a new intake manifold. Again dollars.

If I were you, and I'm shooting in the dark here. If you don't have allot of money to spend and you don't know how the expected results will turn out for you, I would be conservative on my changes. A camshaft that has been previously recommended be purchased with lifters, installed, tuned and test driven for results over a period of time.