If you're going to experiment with engines that are new to stunt, why not experiment with a current production engine like the OS 35AX, ASP S32A, Magnum 46XLS or OS 46AX? That way when you're done you'll have a combination to recommend that anyone can just go out and buy, without having to buy used stuff from eBay and hope that it's good.
I suspect that for a 600sq in model the 46-sized engines would be huge overkill; they're a lot more powerful than the 46LA and may be hard to tone down. In your shoes I'd start with the 35AX.