The rear ones are longer than the rods of the front.
Longest rods for 500 and 250T (478 ccm).
A bit corter for 501 (507 ccm)
The 350 even corter
125 and 250V I've never seen
I think it will be dificult to get the old metallic floats anywhere.
The White ones should do. Have a closer Look at them. Often there are overstanding burrs. You should Pass a fine file to take them off! Otherwise they tend to get stuck
I allways put it spot on overlapping tdc and cut off the belt without any danger to loose the timing. Then put the new belt on as far as I can and then turn the crank. Pressing lightly on the belt it enters itselve after a few turns to his place.
Put he belt on the sprocket as far as you can, 5 mm will do. Then turn the crank several turns with the nut on and a wrench pressing a bit on the belt. It will move to it`s place
Normaly you should take off/cut the belt when the 1st cilinder is on the "overlaping(?)" tdc, not at compression tdc. At that moment there are all 4 valves under presión and the cam will not move, taking off the belt. Normaly the overlapping tdc is when the dot on the upper sprocket is at ...