I agree with PeterC on the CG choice. Use the one that you normally fly your plane with. I built a set of GP 40 size floats (which are 34.5" long by the way) and added them to my Hobbico Superstar. Here's what the instruction manual recommends:

For 60-size Floats: Move the CG 1/2" ahead of the manufacturers recommended CG.

For 40-size Floats: Move the CG 3/8" ahead of the manufacturers recommended CG.

For 20-size Floats: Move the CG 9/32" ahead of the manufacturers recommended CG.

That's actually pretty vague if you think about it, because you're given a range of CG with your airplane. If you like, modify your CG range to 3 1/8" to 3 7/8". I moved mine a bit forward of my normal flying just to be safe and it works fine. In fact I'm sure that leaving it where it was would have been just as fine. Does your current CG fall in that range somewhere? Just use it. The important thing is to locate the step of the float 1/2" rearward from whatever CG you finally decide on (I made a quick plumb bob out of fishing line and a sinker...held it on the wing saddle 1/2" back from my desired CG...put a level on top of the floats, and then slid the fuse around on the gear until the plumb bob pointed at the step...to find my fuselage attachment points). I assume that when you said the "drop in the floats" that you were referring to the "step". Also, remember to have a slight positive incidence from the bottom of your wing to the top of the floats as recommended above.

Hope this helps with your CG question...I'm a newbie at this too so I gave you quotes right out of the instruction manual. In any case, my plane operates great in the water so I'm satisfied they had it right.