If you look at the current posted pictures of the Bellanger "Drag race" headers, you will notice a factory applied flat spot on the #5 or #6 tube. That was added to the design as necessary to fit into the '67 chassis.
These JBA's are not "equal length" tubes so that may be the reason they fit easier?
Even the Hooker "equal length" headers are difficult to install. Hooker themselves say that they are only worth a 5% gain over the standard unequal length, so the suggestion from me would be to ask yourself is that worth the additional difficulties involved to go equal length.
That also would be 5% on the header gain, not on the entire engine. So maybe at 7,000 rpm you might be gaining another 5hp?