drum brakes

hey guys ok so the question is about rear drums. i just put a 318 904 8.75 combination in my70 dart for now, and the 8.75 didnt have drums on it, but it has the 5 on 4 bolt pattern axles. i was told that i should upgrade to the 10" drum for braking because 9" just isnt really that great for anything other than a /6? i am planning on putting discs on the front sooner or later, but for now i just need help with the rear drums. so anyways, i went to an auto parts store today, and asked about 10" drums with backing plates, and they pulled out their book and it said the 10" wasnt on a 70 dart, and that they only started using 10" in 73, and being in 73 they used the 5 on 4.5 bolt pattern.
so my question is: is there anything i can get a 10" drum with 5 on 4 bolt pattern off of? that i can put on my dart? or will i be fine using a 9" drum considering the front brakes do somewhere around 80% of the braking load anyways?