Mary,
For what it is worth I'll give you my 2 cents.

In general the cat rig mains are flattter in the head and a bit fuller in the bottom section.
If you get a main cut for sloop rig sailing you can flatten out the top with stiffer battens and get a bit more shape in the bottom by easing the foot a little bit more than you would when sailing with a jib. It may not be as good as a sail purpose cut but it seems close.
I have not had much chance to sail over the last couple of seasons so what I'm about to say may change when I get back on the water again in ernest (but I doubt it). I have purchased a purpose built cat rigged mainsail and I can make my sloop cut main sail work much better. I'll continue to work on getting the cat rigged sail diallled in but the difference is so small I'd be better off had I bought a sloop sail.
I agree with Wouter. Get a sloop cut mainsail and learn to tune it for cat rigged sailing.
Regards,
Phill

I know that the voices in my head aint real,
but they have some pretty good ideas.
There is no such thing as a quick fix and I've never had free lunch!