I would try your car seat out in the van to see how well they fit. I just traded my Windstar in for a new Town and Country and found that while my Britax Marathon fit well in my Windstar it was hard to get a tight fit in the Town and Country(these are basically the same as a Dodge Caravan, just a step up). One car seat that I was able to fit well in my new van, my Accord, hubby's Camry, and Windstar was the Evenflo Triumph Advance for infant through 50lbs. Although it is a big car seat it has a small base and faces both forward and backwards with a nice fit. Also adjusting the straps is simple all the way around, for the shoulders it simply slides up and down, this is great if you sometimes use the seat for different children. To tighten and loosen the staps it is a simple turn of the knob on the side of the seat and to loosen it is the same spot but a little push button latch that you push and then pull the slack up from the part of the seat where the baby's hiney would be, where the straps go down in the bottom of the seat. If you are looking for a booster w/the 5pt harness for older children I love the Graco Nautalis, I have never had a problem with getting a tight fit and the shoulder harness can be used up to 65lbs. The Britax Frontier isn't a bad fit in my Accord but is a little harder to get a good fit in the vans. There are two ways that they allow you to place the seatbelt if you don't want to use the latch system. You can also use this seat up to 80lbs with the harness so it will pretty much get you through 8-9yrs old. Hope this helps, although I don't have those exact vans I have some experience with using different seats in various cars.