Yeah, best solution is T-34/76 upgun, maybe unlock after T3 and T4 built.
T3 and T4 are combined at 240 fuel, Panther comes in at far less than that.
If you build every tier for Ostheer, you get the following costs:
10 + 15 + 25 + 30 = 80
45 + 55 + 60 = 150
That adds up to 230 fuel, for teching the entire faction. (Don't forget the 20 starting fuel) Assuming your upgun idea too would cost fuel, I think you see why this needs be thought more thoroughly.