1970 Schwinn Tandem - How to move rear seat further away from handlebars
Aww man I'm jealous - can you please add a side-photo of the whole bike?    

70's tandems were renouned for being quite short, and most of the length is lost in the stoker's area.  Mine's a 80's model and one of the touted features is "2 inches more stoker room!"  So if you want the full 70's look, embrace the aero advantage of being so close.  And don't eat gas-inducing foods before a ride!

So you've done the first thing, of replacing her saddle.  Next is to look for a seat post with a bit more set-back.  Make sure you get the right diameter, it could be a 25.4mm pole or a 26.something odd.  Use a vernier to measure, and read all over it for an engraved measurement.



Handlebar style - I'm not sure what handlebars you have, but it may be possible to rotate them, or replace them.  I'm planning on putting some BMX style bars on my tandem for my 12 year old, because he dislikes the dropbars.

You could also replace the bars with cut-down flats.  The stoker doesn't (shouldn't!) be trying to steer, so the handlebars are as handles only.  

As an aside, your stoker doesn't get to see any of the road ahead, so they get all the bumps without being able to anticipate.  So consider a seat post with a bit of built-in suspension.   Your stoker will totally appreciate this.



This style is great, but reasonably pricey.  It induces a setback when still and a little more when compressed.  Photo is a Cane Creek Thudbuster, but there are many others.



This one doesn't have a name, but its the cheaper tube-in-tube sort.  It does take the edge off, but doesn't give you any setback.