Sorry for the dumb question about a window. Of course it has a window - I was just in the middle of my response and did not take the time to go back and check the photos as my post evolved.
I too have a homemade saw which has both of the arbor bearings perched outside the saw tank (like yours), and braced similarly. I might guess that the space between the bearings affects the sensitivity of the alignment in terms of aligning the blade with the sled movement trajectory. I will say that having the pulley for the belt outside of both bearings will allow you to change your belt easily, whereas I have bearings on each side of the pulley (and hence the belt, making belt changes a bit more troublesome).
I have limited experience with gravity feeds, but would suggest that you determine it considering the speed with which your saw cuts the rocks YOU are cutting. Too much weight will cause the sled to crowd the blade (i.e. push faster than the saw can or wants to cut creating stress or heat. Slab saws like to cut slowly and when they do they cut for years on the same blade. The big question is how to determine that the saw cut is still progressing and not just sitting in gentle contact with the blade. For that you can listen carefully and or rig a ruler beside the weight and watch it continue to drop with time (perhaps a flag visible through the window)? I'm not really kidding here because even tho I can usually tell the saw is happily cutting, once in awhile when the rock slips in the vice or the glue gives way on the stub, the rock does just sit in loose but in noncontact with the blade and it hard to tell it other than it doesn't advance as expected, and that's with a power drive. It would be even harder to evaluate with a gravity feed where every rock is slightly different (at least until you are attuned to the sounds and speed of your saw setup). Enjoy the saw. Its unique. You might consider water as a weight (it would be easy to add or remove weight for fine tuning a cut).