It is said that you buy what you want, and breed what you get.
As has been mentioned you have a large beef x cow as a house cow! I understand that you've now built up a relationship with her and probably wouldn't swap her for the world. But why devalue any potentially good calf she may have by crossing it with a dairy breed? At best you'll end up with another massive heifer which is still not ideal as a milk cow. At worst you'll end up with a bull calf with all the conformation and desirability of a greyhound as far as the meat trade is concerned. And lets face it - that's where he will be destined.
So - why not put her to a decent beef breed and get a decent calf, whether male or female? You could then, when she's calved, as she's presumably quite amenable, get a fresh guernsey calf from a milking herd to put on her as well. That way you could still rear the calf as your next house cow, and it would be much smaller and easier to manage and not be eating its head off just to maintain its large size.