Dual exhaust on a 70 should be pretty easy.  The elbow castings that hold the muffler separate from the exhaust manifold and leave two 2 bolt flange ports.

Ours (dad's tractor-my prodding) were threaded for pipe and didn't use any mufflers.  Match the pipe or tube size to the maniflold opening, and you should be good to GO!

The gasket is the template for the manifold flange.