Most of the "plans" for Greenland style paddles assume that your kayak is just slightly wider than your hips and call for a paddle-shaft of about shoulder width or so (this is a highly personal measurement). If your kayak is much wider than this and/or has a large amount of foredeck volume, then you will need to lengthen the paddle shaft to at least the beam measurement, to keep from bashing your knuckles on the deck. Since this will shorten the paddle blades, you will probably have to lengthen the overall paddle as well. Sorry, I have no formula for you, I would expect that you would need to experiment, by trial and error. If you are interested in traditional paddles then you might wish to research and build a paddle used by kayakers who paddled kayaks similar in shape to yours (there is a tremendous amount of variation in native kayak design). A Greenland paddle/kayak is a tuned system and there is no guarantee that it will perform well if scaled significantly beyond its original dimensions.
As for your current paddle, you might also gain relief by adding a slight sliding component to your stroke, useful for dealing with too short a loom.