Pardon my ignorance in this regard, however; are front JL8 brackets not the same as 67-68 (4piston) disc brake brackets?
IF the spindle were different, then just use a 67-68 spindle and 67-68 front calipers, whatever you are building is not a restoration, correct?
I mean you're not trying to pass whatever you are doing off as original, I am assuming... and thus hoping I am not making an arse of myself nor of you.