Obturation advice?

[macadoo]

I started this re-treatment today.  I don't know anything about the original RCT on the tooth…this elderly patient estimates it was done "probably 15 years ago".  I put her on an antibiotic back in 2013, when she had a draining fistula and some discomfort at the time.  I had referred her to our endodontist, but she never went b/c she "only trusts me" and doesn't want another dentist working on her.  

So, I've got a scope now and have been doing the majority of the endo in our practice for a while now.  When she came in for recall last week, I told her I'd give it a shot, though it would only be the second re-treatment I've done.  

Anyway, I got the GP cleared out today pretty well, using Edge XR retreatment files and chloroform.  I'm not crazy about the speck of GP that got pushed out the apex, but hopefully that won't affect the outcome.  I gauged the palatal canal at .45 mm.  I had to pull out the old doc's collection of large hand files, but the largest was a #70, which went about a mm long in the buccal canal before binding at the apex.  So, the existing apical preparation should be around a #75 in that canal.  I'm curious what everyone's next step would be.  I've got the tooth sitting in Ca(OH)2 now.  Is there any point in further enlarging these canals?  Would you obturate a canal that size with GP, or obturate with MTA since this is likely the last NSRCT for this tooth (or should we leave the option of apical surgery open)?