To install 3.55 gears you would have to replace the carrier as well... Buick used 3 different carriers depending on gear ratio:
1) 2.56 and 2.78
2) 2.93, 3.08, 3.23
3) 3.55, 3.90 and 4.30
There are aftermarket posi carriers avail. Jim at JDrace.com is a Buick guy that can set you up.
Moving the crossmember back isn't too complicated. Best to weld on new frame brackets, but it can be as simple as bolting 2 pieces of 2.5" (approx) angle iron to the frame.
A shorter drive shaft will be needed. Parking brake cables need to be modified or replaced.
A shifter conversion is avail from:
http://shiftworks.com/buick.htm