I'm planning to upgrade my rims from 305mm to 349mm. However, it seems that most v-brakes have a max distance of 1" from the fork boss to the center of the pad. Are there v-brakes that have a reach of say, 48mm?
Assuming you do mean reach as distance from the center bolt (for mounting a caliper brake) to the middle of the brake shoe:
For a center mount brake (e.g. many caliper brakes), the reach is determined by the length of the brake arms, and you can get different reaches easily by swapping to a longer or shorter armed brake (or make a drop bolt allowing you to use a shorter reach brake for when you need longer reach).
However, brakes mounted on bosses like V-brakes have their "reach" determined by the position of the bosses in addition to the brake. This can vary from frame to frame (or fork to fork) a bit, but usually not beyond the 10-15 mm or so allowed for brake pad adjustment. The exception is probably the Paul's Moto BMX V-brake:
which is sometimes used for 26" -> 700c conversions and might be enough for your purposes if you can find one.
Another way to do the swap is with a brake post conversion adapter -- a piece of metal which mounts at the brake posts to provide brake posts further up.
Plus a V-brake with sufficient adjustment.
Another option I've seen (which I wouldn't recommend) is people taking a rectangle of metal, mounting the brake pad at the top of the rectangle, then attaching the bottom of the metal to where the brake pad is normally mounted. This likely results in a good loss of braking power, plus you have to make sure the mount is very secure.