Hi Metalguy,
I bought an AYA 12ga. the same as you describe and I sent pictures of it to W.Ellis Brown and talked to him about it's suitability of conversion to a double rifle. I was talking about making a .450 BPE or .500 BPE and he thought it would be more than adequate for that conversion. It might be a bit large converted to the rounds you are speaking of but it should easily handle them. You would have a nice DR in the end though.
Good luck and post pics of your work.
Yukon.