Should it? I don't think it would be that bad to have 76mm and E8 identical and have E8 be a more expensive, better armored and faster tank.
I guess it's debatable. I mean Easy8 as a slightly different 76MM would work fine but personally I feel like it's more interesting and fun to play with distinct units and not "this unit has 10 better penetration, has more health and fires .5 seconds slower but is otherwise exactly the same". (Obviously not exact stats but you get more point). Thats why I brought up Veterency - Easy 8 has exact same veterancy as M4A3 (Barring the Vet 1 Change). 76MM is slightly different with frontloaded reload % at Vet 2 and bonus Penetration at Vet 3. If Easy 8 is going to be kind of same-y then I think you could at least tweak Vet to emphasize it's relative strengths to M4A3 and 76MM so that while Vet 0 76MM and E8 may feel similar that you actually notice a difference at Vet 2 and 3.