WTF with the 5 x 4 BC and getting axles?

So what is the verdict. I am in the process of building a 70 swinger 340 stroker and hope to be putting out about 500 HP when I get it finished. Will the small bolt pattern support that HP and are they still available? I really would like to keep my SMB rallyes as I am planning to have a machine shop split the rims and weld in some width for a much wider rear tire as I am moving the springs in to the frame rails. Has anyone gone through this process of widening the Rallyes if so let me know how they turned out.

There are still alot of the build options that I have not decided on yet so some things are still up in the air for me, but I want to make the right choice during the build not after, thanks guys!!