If you give the venture_A have a -15% armor on its base stats pre-hullmod buff, it brings the venture_A up to the same armor rating as the Dominator, which is explicitly stated in-codex as being the apex of cruiser combat armor. You could even add a little line in the description that mentions this fact, along the lines of "Internal size restrictions mean that up-arming the Venture requires removing some of its less-efficient original armor, but not more than is added by the modernizing upgrade."
Also, since the _A is a standardized upgrade line which you're having to downgrade the base slightly to accommodate for the up-arming bringing it above reason, it would be reasonable to slightly upgrade its engines along with the base downgrade of the armor. Since it's a 15% armor buff that brings the _A class to the Dominator's armor, its base buff could be something to the tune of a 15% burn-speed increase, which with the venture's base of 7, incidentally brings it up to... 8
which, if a player wanted to flagship the venture_A and get its fleet to a burn of 9, also implicitly reducing its fuel cost by negating the inherent need for a tug. Which, not needing a tug is in line for a militarized vessel