how to spider gears

no,the LBP axles are shorter by about an inch than the SBP.
I learned this all the hard way changing my '66 cuda to LBP about 4 Mos. ago.
However all the internals are the same.That is why I said you could change your 3rd member between housings by only re setting the backlash.
But you must keep the ring/pinion together as a set.
The spiders can be replaced withought removing the 3rd member from the housing.I assume your "open' diff is the broken one?(earlier offer was serious)