What kind of rim tape do you have in there? If it is the rubber strip type, pull it out, and then try mounting the tire, often that frees up a huge amount of space, and you can use some filament tape for your rim tape. Of course these days it is rare to have those heavy liners on better bikes.
I just want to warn you that you can have a tire change as many as 4 times a day, in my worst sad sack experience. One of my criterion for any tire/wheel combo is that it changes easily. I can pretty much strip my 700Cs by hand, with minimal effort, though I use a quick stick because it is easier still. My current combo is Schwalbe marathon slicks on Alex DH22 rims.