If you are going to the 4 1/2", the 8 1/4" axle would be the cheapest and plenty strong for the slant 6. You'll have to get axles for the 8 3/4 if you go that way. Plus the 8 3/4" is much more expensive in the first place.
We are planning a V8 conversion. Only a 318 so yes the 8 1/4" is actually what I'm looking for but really just want I can find and in my price range. I appreciate all the advice from everyone!