If you go with the Econoline, the 351W is probably the best motor for the average user. The 351M isn't bad, but it pales in comparison to the 351W. The 351W is physically smaller, so it will be easier to work on in the tight confines of a van. The 351W will also probably have more power and parts are far and away more available than the 351M. Still easy to get parts for cheap for the 351M just not like the 351W.
The 400M is a bit of a pig on fuel, but that is my opinion. It does have good power.
Some people say the 460 will get the same fuel mileage as the 351, but I highly, highly disagree. The 460 is extremely reliable and parts are cheap and super easy to get, but it is a big motor and hard to work on in the tight space of a van. My opinion on fuel mileage is that is gets horrible mileage. 8 mpg if you are doing good. You can get maybe 17 or 18 mpg with a 351. The 460 should be considered if you really need the power with fuel mileage less of a concern.
A 302 would be fine, too, as long as you aren't working a stock 302 too hard.
Oh, the 300 six cylinder is a great motor. Very durable, very easy to work on, has decent power especially on the low end, but the fuel mileage (16 mpg) will be no better than a 351. Unless you are towing or need to go 75 all the time, this motor is certainly one to consider.
Hope this helps.