Yea, it depends a lot on which van you get. It sounds like cost of gas is very important too you, but if you do the math I think you will find that a full-size van doesnt save as much money as you might think. I know those 460's are super thirsty (8 MPG range in the best of conditions) but a mid 80's full sized van with a V8 will only get you 10 MPG at best. Pay more for a fuel injected 90's van and you might get up too 12-13 on average. Find a 6-cyl van and you can work your way into the 15-16 MPG range. If you really want to down size, a mini-van (Ford Aerostar or equal) will get you into the 20's (my Ford got 26 MPG) but at a huge loss of living space.
But, all this will save you only a few hundred dollars over the course of an entire year, even if you drive 12,000 miles a year. Especially right now with gas prices being so low.
You have a beast of a motor in that thing, it can easily tow a small car. Park the C and take the small car out for sight seeing or errands an you save a some money right there (after the initial cost of a car and towing mechanisms