Ballantine IPA was a 6-row based beer. Imported malt is going to give the beer a different malt profile.
Not in my experience...it all depends how you treat it.
in the 30+ years I've been brewing a BIPA clone, I've freely substituted the 6 Row with a variety of imported pale ale malts depending upon what was available to me. Any differences have been very subtle (if noticeable at all). Around 20 years ago I even started adding in a bit of imported Munich malt as well; it actually wasn't until I did so that I finally hit upon the taste I was looking for, remembering the boatloads
of the original Newark brewed Bally IPA I was consuming in my youth (even though it's fairly certain that the original never used any Munich malt, in my setup and my procedures it proved to be the missing link in the taste).
Clusters and Bullion (or Brewer's Gold) are, of course, definite musts
in recreating the original. The recent revival of the product, while quite good, fails somewhat in the 'hop character' department by using different varieties, some of which introduce citrus notes that shouldn't be there.
Also...if you want to get closer to the yeast used in the original, don't
use 1056, US04, or any of the variants of that strain: instead, use Bry97 or ECY "Old Newark Ale".