Bac,

You will not need a crank puller for the Ultegra crank arms. Here is Shimano's diagram of your setup; http://bike.shimano.com/product_imag...FC_6503_EV.pdf
All you need to do is back the crank arm fixing bolt out with the crank arm cap in place and it acts as it's own crank puller. As long as you use Ultegra crank arms you will not have to replace the bottom bracket.

You will want to remove the crank arm cap before putting the setup back on so that you can see inside to make sure you have the splines lined up properly before tightening the crank arm bolt down with your wrench.

I am considering doing the same thing with my bikes. I am also 5'8" although my inseam measures about 32.6 inches. The Fit Kit recommends that I have 170 mm cranks. On my road bike I have 172.5 and on my mountain bike I have 175. When I get on my mountain bike after riding my road bike I notice a really big difference. I just haven't decided if I want to go to a 172.5 on the MTB and keep them the same or switch them both to 170.

Let us know how it works if you try it.