How'd you get an E-250 with only a V6? I assume you are talking about pulling a trailer right?
Well, this should be pretty simple. Find the towing capacity of the Van. The bikes are roughly 450lbs each so that's 1800lbs of Bike. Then the trailer is going to be between 1200-2000lbs depending on what you have. So you are looking at between 3000lbs - 4000lbs total. If that falls within the guidelines for the towing capacity, your good to go.
If you are putting 1 - 2 bikes in the van, you can cut your towing weight a bit. Remember though, extra weight for the gear gas and what not you bring.
FYI: I used to have an E-150 with a v8 I used to tow upwards of 5000lbs with no problems.