As for the subject of the 7.5cm KwK40 L/43 versus the 45mm glacis of the T-34:
The 7.5cm KwK40 L/43 was, as all German guns, extensively tested and found capable of the following average performance against a 250-265 BHN RHA target plate tilted at an angle of 30 degrees from the vertical:
PzGr.39 (6.8 kg, APCBC-HE, 740 m/s):
100m = 99mm @ 30 deg
500m = 91mm @ 30 deg
1,000m = 81mm @ 30 deg
1,500m = 72mm @ 30 deg
2,000m = 63mm @ 30 deg
In addition to this it is noted by Livingston & Bird, that the German test plates were of unusually high quality throughout the war, which would mean performance was even greater in practice versus Allied armour. (Worth keeping in mind) The quality & nose hardness of German AP projectiles were also higher than those of the other major countries.
T-34 front upper hull glacis specifications:
Actual thickness: 45mm
Slope/Tilt: 60 degrees from the vertical
Straight line relative thickness: 90mm
Type: Rolled Homogeneous Armour
Brinell Hardness Number: 420-430 BHN
With all this information in mind I would make an educated guess that straight on, with no lateral angle, the 7.5cm KwK40 L/43 was effective with the PzGr.39 against the T-34's 45mm glacis plate out to between 500 & 600 meters.