Amazingly l just saw your post but not sure how much l can help off the top of my head but here goes. First thing to say is that if the tubes protrude into the BB tube then you have a problem. The Zenith sealed unit l used is a tight-ish fit and l had to spend quite a bit of time carefully filing away just one of the tube ends which protruded, it is very thin but hard metal. I will get answers to your other questions ( l think it was 68mm but will check) . I got it from Velo Solo and they may be able to advise axle length for your Suntour cranks. You cant just swap axels in the sealed unit as far as l know. You could always use sealed bearings with your existing axel which is what l originally did except that having got everything nicely installed and working a treat, l went and snapped one of the crank retaining bolts when l tightened it very neatly flush with the end of the axel
so start from the beginning again.
Sealed bearings are maybe a bit trickier to install but quite a bit cheaper than a sealed unit. Hope all this helps !