Transformer question

[ranchak]

I'm building the Shaka Tube and it requires 12.5VAC. I have this transformer:

http://www.radioshack.com/product/index.jsp?parentPage=family&summary=summary&techSpecs=techSpecs&currentTab=features&cp=2032058.2032230.2032277&custRatings=custRatings&features=features&accessories=accessories&productId=2102702&support=support&tab=summary

It's 12.6VAC CT, my question is can I use the two output leads and attach one to the negative input and the other to the positive input? (I know you get the rated voltage by doing this, I am not sure if this the appropriate way to wire the tranny) Should I attach the center tap to ground? I get actually 14.6VAC, as a rule of thumb, how much larger should your capacitors voltage rating be to be on the safe side?