301 a domain and using the content from that domain

I've got a domain that and I want to 301 to a new domain I bought and use the content from the original domain on the new domain.

Eg 301 olddomain.com to newdomain.com and use the content from olddomain.com on newdomain.com

The reason is that newdomain.com is a far better domain name and can be branded and even though olddomain.com is ranking a little, it's not making any cash but I still don't want to lose the link juice/domain authority from olddomain.com because it is ranking and has some links.

The thing that I'm most concerned about is duplicate content and losing link juice/domain authority. I don't want any duplicate content issues or anything like that. If I go onto GWMT for olddomain.com and delete the content from cache wouldn't that mean that olddomain.com's domain authority would be crushed so then if I 301'd it, it wouldn't have as much link juice?

What's the best/safest/quickest way of doing it properly?

Honestly, I would just clone the website, put it into the new domain and after that I would 301 every post to its respective page. It's basically a migration of a kind, shouldn't be any problem with google.