i have done both routes ,i first used j2 rear subframe ,it was a nice simple task,the j frame is allot easier to do by far,using the b1h frame doesn't bolt straight in ,i have had mine modified to fit my j frame ,if you know a welder who can weld it all up then is an easy job i'm in the hgv commercial truck trade so its all a piece of piss for me as i have friends in all theses trades.the reason i went b1h route was i wanted to fit the b1h battery box as well and wanted to plate the outer part of frame aswell so that's the only reason i went down this route ,if you are happy with the j battery box i would use the j subframe look for scoobs thred he done a good write up on it ,i used his thread