Thanks for the comment - I think these drums look pretty awesome as well!I am still struggling with whether or not I should keep them in their original configuration (for the sake of collector value) or beef this kit up with another rack tom for the fun of playing it the way I am used to.Let me know what you think.
There are a few easy ways to make your set a double tom set without sacrificing originality (permanently).
1. You can buy (from ebay, or drum forums) the setomatic double stubby thing seen here.
http://cgi.ebay.com/Slingerland-1975-Black-Chrome-22-16-13-12-99-95-NR_W0QQitemZ250367508039QQihZ015QQcategoryZ10175QQssPageNameZWDVWQQrdZ1QQcmdZViewItem
It the top piece retrofits your single arm...just don't misplace your stuff when you do that. This was pretty standard for Slingerland in the '70s. They've become pretty unpopular because the tom positioning is real limited...tilt the right tom up, the left goes down...
2. You could spend a lot more on a later Super setomatic mount which will also fit on the ball you have. It works well, but finding what you'll need (the big "block" and another single arm) will cost a bit.
Here's the "block" part...that goes on the ball and then you put a single arm in each small hole...
http://cgi.ebay.com/Slingerland-Super-set-o-matic-Tom-Mount-Parts-Project_W0QQitemZ250367229330QQihZ015QQcategoryZ47089QQssPageNameZWDVWQQrdZ1QQcmdZViewItem
2 easier, cheaper and more flexible ways would be to either put one tom on a snare stand...put the first tom on the stand and use the arm for the second tom, OR, put a RIMS type thing on the extra tom, and hang it with a multi clamp from a cymbal arm.