You can't use the 5-point harness past 40 lb...
If the child is actually over 40 lb (not 39 lb or less), then you cannot put that child in the harnessed seat. You can, however, use it right up to that 40 lb limit (and, a few oz more won't make a difference, just don't push it).
Some boosters are narrower than others. Assuming there is head support, you could consider the Britax Star Riser (
www.britaxusa.com/ the Britax site seems to be malfunctioning right now, but should be back up later today or tomorrow -- click "Boosters"). It adjusts to be very narrow.
Some seats also "mesh" together better than others. You could go to Toys R Us and try different combinations out in your vehicle and see how they work.
If the smallest child really is 40+ lb, then you'll need to change his/her restraint. One option is the Fisher Price Futura. If there is a Big Lots in your area, try calling them to see if they have any left. This seat has been discontinued, but it goes to 60 lb with the harness. It comes in a 5-point and overhead shield version. The 5-point is pretty rare now.
Britax is coming out with a new seat in Sept/Oct (probably later, rather than sooner) which will go to 65 lb with the harness, called the Marathon. It will cost over $200.
Britax currently has available the Super Elite. It goes to 80 lb with the harness. It's a very large seat, and I suggest you try it out before you buy or make sure you get it from somewhere with a good return policy. Depending on how it meshes with the boosters, it might be a good option. It runs $200 as well. See
pages.ivillage.com/momika...s/id2.html (The Traveler Plus uses the same shell as the Super Elite. She has 4 pages of pictures.) And here's another Super Elite page.
www.angelfire.com/mi3/ter...elite.html
Robert also mentioned the Britax Laptop. It's a unique "seat". See these three pages for more.
www.angelfire.com/wa3/isakswings/lt.html pages.ivillage.com/momika...s/id7.html and
justhandy.com/laptop.htm
Finally, if it does become necessary to put someone in the front seat, put the largest child there in a correct fitting booster and slide the seat as far back as it will go. The back seat is safer, but an appropriately restrained child in the front is still safer than an improperly restrained one in the back.