The height limits on car seats are just guidelines, but the weight limits are set in stone. A rear facing car seat is outgrown when the child goes over the weight limit or there is less than one inch of hard shell above the childs head.
The reason height limit is just a guideline in car seats is because each child is build differently. My boys, for example, my oldest is all legs and at 6yrs old was on the same harness slot that my now 4yr old is on in the Britax Regent, because my youngest has a very tall torso.
There is, however, a height minimum to forward face, and most seats is 34" tall. So until she hits that she must remain rear facing.
Anyway, if you are looking for a convertible car seat, any will do because if you can fit the safeseat in the back, you can fit a convertible because they can sit more upright and often take up less room than the infant seat.
Cosco Scenera ($50) will rear face to 35lbs, forward face to 40lbs. Fits most kids to age 3/4 in the harness. Good bare bones seat, very safe.
Evenflo Triumph ADVANCE ($130) will rear face to 35lbs, forward face to 50lbs. Fits most kids in th harness to age 5.
Graco MyRide 65 will rear face to 40lbs, forward face to 65lbs. Fits most kids in the harness to age 6. This is a wonderful seat, great for extended rear facing, fits older kids well with lots of leg room.
The AAP recommends children remain rear facing until the rear facing weight limits of their convertible car seat, or at least 2yrs and 30lbs. You have a Boulevard in your other car, this is a GREAT seat for extended rear facing. Please google EXTENDED REAR FACING and JOELS JOURNEY for more information. Legs touching the seat is no problem, there has never been a documented case of broken legs while rear facing, but there are children who die from internal decapitation or are severely injured from spinal cord overstretching every day.
The harness should be at or below the shoulders for rear facing. THe harness should lay flattot the body, and there should be no bulky clothing (no winter jackets, buntings, etc) worn on the car seat. The chest clip should be level with the armpits.
Edited to add that the TrueFit is a great seat, but the headrest must be added at 22lbs, or when the childs head is at the red line, and at 29" tall and 20lbs, she's close enough to those limits that the headrest would likely have to be added, making its short profile without the headrest moot.