the b1h battery box runs all under subframe all flush with the seat frame, the j series hangs out the bottom of it which i didn't like ,down to preference really,the seat is a bit of a mod as well,but is worth doing as they look shit with b1h seat as they dont follow tank line ,i used the front part of my seat j series and the rear of b1h seat and bonded together have a look at scoobs thread ,thats the one i followed,there aint much work to use j seat frame there is no welding involved jast brackets and a bit of chopping,i have just finished my b1h seat subframe project this week i only need to get seat unit sprayed then done,i have some pictures of the b1h subframe all modified and cut /welded if you do need any pics let me know ,but i will say the j frame is easier to do than the b frame