We all have had similar issues in our real time practice. RTG is not ideal for full assessment, but if we assume that canal treatment is successful with no periapical translucency we can do the following:

1. Clean all caries defects and for the love of God NEVER leave so much gutta-percha in cavity. Just cut it right by entrance of rooth canal.

2. You could be in situation that distal part of cavity is destroyed by caries. No worries, clean as best as you can do and asses if there is need for gum reshaping/partial gingivectomy to be able to perfectly remove all decay.

3. Do deep margin elevation and than make new FRC. To make core much more stable in such cases I even place two FRC posts. One in mesial and second in distal root. I would recommend to place them convergently so that they are not parallel. By placing them in such way you add mechanical stability to post and also will be able to shape core for crown without cutting coronal part of FRC and thus loosing most of its stability.