At 6 ft tall you should probably be on a 56-58 cm frame.

I dont think you will be able to replace a 39 tooth ring with a 34. theres different bolt patterns between a standard crank and a compact crank, you might be able to find one but be careful to get the right bcd.

It would probably be easier to replace your cassette but your rear deraileur may have a limit as to how large you can go. eg.my campy chorus short cage maxes out at 26 teeth, 27 and it rubs. you will need 2 special tools to do this or your lbs will for a fee.