If the Cowboys do sign Jason Peters, then I think the most logical way to play Peters is to have him at LT and Tyler Smith at LG. There are a few reasons for this. The first reason is because Tyler Smith was used at LG during training camp and the preseason. Part of the reason is because he's a bit raw for LT. I think starting him off at LG allows him to be shielded while he learns proper blocking in the NFL. By having Jason Peters, this can continue.
The second reason is because Jason Peters is a very similar type of tackle to Smith. So, by having Peters at LT with the knowledge that Peters has from having played such a long time in the NFL, Tyler Smith can learn from a highly experienced mentor how to use his strength at LT when blocking. Essentially, having Peters here allows us to mold a younger Jason Peters. This would allow us to help Smith to transition a bit more easily to LT when that time comes.
Finally, Peters, while with the Bears, was still very good at LT, and I think he is probably a bit more comfortable at LT. So, he'd be at a spot where he is able to block at his best, this improving our oline by virtue of this.
As such, I think the right move, if the Cowboys sign Jason Peters, is to have Jason Peters at LT and Tyler Smith at LG. At least to start off.
