Sorry! Another which convertible thread!
Is there a reason you are looking for a convertible for your forward-facing child? It would make more sense, and you would get more growth room, to purchase a combination harness-to-booster seat like the Graco Nautilus or Britax Frontier. Neither are narrow, but both will keep a big kid harnessed to a safe booster age and size (and then the seats make decent high-backed boosters).
The Complete Air states a 40" limit for rearfacing. If that's not a problem for you, it's a fine choice. So is the Radian: with an angle adjuster, it can be installed fairly upright in most vehicles. I would not recommend a MyRide for a very tall child or a TrueFit for a heavy child -- since your child seems to be both, I'd look at other seats.