I cant answer all of your questions but I can start with some of them while others fill in the rest.
1) We can't measure your rear end for you. If you can confirm with 1000% certainty that the rear end originally came out of an A-body then those should be your measurements. (The fact that they are SBP is a good sign that they are) If you can't confirm then you will have to do your best to measure it yourself.
2)To my knowledge although I may be wrong, all stock axles were 30 spline, and the 35 spline axles were aftermarket heavy duty upgrade stuff. Someone will chime in to correct me or confirm.
3) There are 100s of debates of modern green bearings vs timken, it has been beaten to death. On a mild street cruiser you would probably never notice the difference between the 2 but for race applications look up the pros/cons people have posted about them (I personally recently bought axles from Dr. Diff and went with the green bearings because I believe they'll be enough for MY application)
4) BBP axles have different offsets, so they will require new brakes. Dr. Diff sells what you will need, If you plan on going to disk brakes, It would make sense to do that swap now
5) The axles I ordered from him had 10 holes, 5 for press in studs (installed) and 5 for screw in studs. Swapping out studs for different lengths is just a matter of pulling the axle and swapping what you need, I am a big fan of that feature.
6) Also debatable, what kind of power do you plan to run? Transmission? street use or heavy drag use? In theory your 8.75 should handle just about anything your 440 can do on the street with a reasonable tire (not crazy wide or sticky) Many people upgrade do Dana 60 if they plan on racing on big blocks and for peace of mind. A 8.75 can survive longer behind an automatic trans based on how power is sent to the back but manual transmissions can be brutal on drive trains and sometimes require more beef.
All that being said Mopar teams raced with 8.75s behind Hemi's for years before they decided to put Dana 60s in their cars
This issue, again, has been beaten to death in 100s of threads on this site
