that depends on the 9" housing you are using or how you are building it. You see if you have or are building a small bearing 9" then I would say yes you can put the 8" backing plates brakes and all on the 9" housing. or if you are building a 9" from a large bearing housing then if you choose to you can put the 8" housing ends (same as small 9"
on the large bearing housing and then use all of the 8" brakes then also. I hope this makes sense to you. You see to me it is all possible as I have and do build custom 9" rears and have done it both ways with out any trouble at all.