No, in that case adding some reinforcement is a good idea. I like to be able to hang doors and trial fit when doing something like that so welding a piece of tubing across the door opening on the inside pinch weld at the firewall and at the quarter is a great way to go. You use something like 1" square tubing with a nice little bead welding it on each end. When you are done you just cut the weld with cut off disc when you are done. While you are changing the rocker you can hang the door and check fit before welding, or even during welding.
I am BIG on trial fitting parts, it is time very well spent.