Are you sure SB bottom retainer will hold BB spring? Isn't its diameter smaller than diameter of BB spring? And the same with collar.
Why there should be a less room for a spring? Sorry but I can't imagine that.
I found on Petit that you need BB bottom retainer and probably a collar from Traxxas (TRA3768) which you simply put on SB collar, is anybody using it like that? And yes, in this configuration you have slightly less space for spring but you can adjust it with moving SB collar a bit upwards.