Hi! I'm a Certified Child Passenger Safety Technician and I can point you in the right direction for your little stringbean. The Sunshine Kids Radian ($200) has the highest shell height to accomodate RFing for a longer baby. The 65lb limit Britax seats($230-$300) and Evenflo Triumph Advance (make sure it's Advance!)($120) are also good choices for long babies.
The American Academy of Pediatrics recommends rear-facing to the limits of your convertible seat, which is typically 30-35lbs. My 3½yo is 25lbs, rear-facing and happy as a clam.
Also, an infant seat is outgrown when the top of the head is less than an inch from the hard shell at the top of the seat OR the baby is over the weight limit dressed and shoed (not naked weight). Legs touching the seatback is irrelevant and there are no documented cases of leg injuries due to rear-facing.
I hope this helps! http://www.car-safety.org/rearface.html