A couple of things to consider:
You are right about the Baby Trend. Because the handle MUST be down when it's rear facing, it takes up a lot of room front to back even though over all it's not that spacious of an infant seat.
I would not suggest a convertible for your baby.
1. A Britax Boulevard is not safe for a newborn and you can't use it that way. The bottom most slot is more than 10" from the seat pan. Newborns usually have on average 7" torsos. Straps need to come from at or below the shoulders. See the conflict?
2. A convertible, with a few exceptions, is going to make your problem worse, not better. Newborns need to be reclined to a full 45 degrees. Convertibles are substantially bigger than infant carriers, and while they take up less room front to back for an older baby or rear facing toddler, when a convertible is reclined at 45 degrees the front passenger can end up in the dashboard.
3. The convertibles that will work at a full recline are not going to be cheap. First, there is the Combi Coccoro (~200). It's a cool product, but it's a niche one. Basically it will work rear facing only a little longer than the big infant seats, and then it moves to forward facing.
Problem is, kids need to be rear facing a LOT longer than that. 2 years is a bare minimum. 3-4 is the ideal. Are you familiar with the push to keep toddlers out of forward facing seats? For a lot of reasons, forward facing a 1-2 year old child is simply not safe. So you'd end up dropping 200 on an Coccoro and then having to buy a new convertible down the line.
A seat that I would DEFINITELY recommend for you, that would NOT be cheap, but will solve all your problems, would be the Maxi Cosi Pria 70. It is a very large convertible that will keep children rear facing to 3-4 years old, as they should be. The nice thing about it? There's an infant insert in the seat that changes the angle WITHIN the shell of the more upright seat. So you install it nice and upright and it doesn't impeded on front passenger room, then insert the infant insert and voila, baby has a 45 degree angle. When baby gets big enough not to need that angle, the insert comes out. (These are not the dangerous after market inserts you buy in stores. This is a solid, crash tested device that is structurally sound and designed for this seat for this purpose.) However, this seat is 250 dollars. If you can swing it, it will keep your child rear facing 3-4 years, and harnessed until 5-6 depending on your child's size.
If you CAN'T swing 250 dollars, my best suggestion would be to buy one of the cheaper infant carriers that is 1. small, 2. fits newborns well, 3. front adjust and 4. Can have the handle up in the car. I would look into the Safety First Comfy Carry http://www.google.com/products/catalog?client=safari&rls=en&q=safety+first+comfy+carry&oe=UTF-8&um=1&ie=UTF-8&tbm=shop&cid=7562946142813420031&sa=X&ei=by9GT4yeAenc0QHA_t24Dg&ved=0CDoQ8wIwAw#ps-sellers.
Then, when your baby outgrows that, use the Boulevard for the baby and buy a combination for the 4 year old.
However, keep in mind that the Boulevard, if you bought it when your 4 year old was a baby, is going to expire BEFORE your baby should go forward facing, so eventually you'll need to replace with a new, larger convertible.