Folks - I need dirrection for a new project.

I bought an example of the aforementioned single shot for $150 as a project seed. Don't know how many of you are familiar with it (as I wasn't before I stumbled on this one), but it's a modestly built and furnished break-open single shot as seen here:

http://ww.gunbroker.com/Auction/ViewItem.asp?Item=80222917

As you can see, it's kind of an ugly little thing, but I've got plans to make some significant cosmetic changes (new mannlicher stock set from scratch, much metal recontouring, etc.) that will hopefully improve things.

The part that I need advice on is the new chambering. The lugs are on a monoblock and it will be very simple to rebarrel and modify the extractor (is for a .22 hornet now). The pressure ballpark we're talking about though is described by it's current chamberings of .222 rem and .22 Savage Hi Power. I'd like to go up in caliber to something that could take whitetail given an ideal situation. What would be the best performance obtainable from a chambering safe in such a rifle. 6.5TCU? 7-30 waters maybe? 357 Maximum? What about those of you familiar with contender rifles and martini cadets... same ballpark?

Thanks for your thoughts.