For a cruiser style bike, 340lbs is not a weight at which you need to consider custom wheels.
If you are truly concerned, take a look at the bike you are thinking of getting, check out the maker of the wheels, get the model/type, and contact them. They will be able to inform you of the max recommended weight, or you may even find it on their website. In any case I think you will find you are well within the limit.
As an example, I was 300lbs not so long ago. I am now down to 280lbs, but even so, I ride around on my cruiser, with my 120lb wife sitting on the padded rack at the back, for hour long rides. So that is 400-420lbs on a standard cruiser, and no problems at all.
To give you another comparison, even the wheels on this bike have a recommended max of about 330lbs, and this is obviously far from a cruiser!
