Not a lot of action on this, so I'll throw out my humble opinions from personal experience:

 

Welding the exhaust system is the only way to go.  I definitely would not use sealant anywhere-it wont last.  Also, the idea of clamping a slip joint so that its removable later dosent work in my experience.  Those muffler clamps with the "U" bolt and the saddle actually crimp the pipes togeather and you will have to beat and pry the snot out of the pipes getting them apart (if you even can) until they're pretty much destroyed.  Much, much quicker to do a clean cut and reweld.  You want a removable bolted joint somewhere in the system, (which you have at the header outlets), weld everything else.

 

Your original joint at the reducer just back from the header 3 bolt flange shouldn't have cracked; something is not right.  You must be hanging too much weight on that joint or maybe it wasn't able to flex with the motor movement.  Cleverly placed hangers are the answer.

 

I'd weld the new joint/reducer on but make sure its not preloaded.  Support the exhaust pipe weight with hangers that can flex side to side a little.

 

Don't worry about welding chrome plating, just grind it off if you cant find a bare unfinished piece.