The T-44 and Panther both have 18 AF hull. The Panther had 80mm, the T-44 had 90mm glacis armour. The T-54 upped that to 100mm, so should get 18 AF. To get a 8 AF at that kind of slope would require the T-54 to have only 40-50mm frontal hull armour. Hull sides at 80mm should get 8 AF. As for structural weakness, I've never heard complaints in that regard. Crews freezing in winter, tight as a virgins arse, rounds rolling about the floor, old fire control, yes, falling apart, no. The IS-3 had problems with front hull armour coming apart at the welds after cross country bouncing and firing shock. That was a complex shape made up from multiple slabs welded together in the IS-3. The T-54 glacis was a big flat ignorant lump of metal with welds only where it met the side and lower glacis plate.
There are 3 turret shapes.
The first was the m1946 (T-54-1) which was a continuation of the T-34/85 and T-44 outline. Indeed distinguishing a T-54 m1946 and a T-44 could be difficult. Only small number produced due to severe reliability issues. I would rate the turret at 18/11 which would give an overall 18/. This version should have Red MP and Soviet MAVN M. The Egyptians have one on display.
The second is the m1949 (T-54-2) which had the familiar inverted frying pan shape but with a noticeable overhang at the rear. The third is what we most commonly associate with the T-54, m1951 (T-54-3), similar to the m1949 but without the overhang. Both I would class as 26/11 for an overall /.
For all above I would treat as100L T,  RoF ("NO IF" not applicable), B(11) (a low ammo B11).
After the m1951 things get messy. The T-54 series went under many upgrades and rebuilding resulting in a early T-54 that might be as good as if not better than a new T-55. So over time the B(11) could become B12 (a few more rounds), allow G (gyro) or better (a WW2 G is a single axis stabilisation, the T-54/55 had 1 axis then 2 axis gyros). Exhaust injection smoke generation was another moderately early addition.
Of course once you get into the late '60s to 70s you have the problem with newer generations of anti-tank rounds. For both WP and NATO guns the advances were quite astounding. To replicate that in ASL you would need date/war/country dependant TK tables. Eg In the 2 US-Iraqi wars the Iraqi T-72 were not regarded as too much of a threat to the M1 as they were using old Soviet rounds from the late '60 and '70s and had been superseded by much more formidable rounds in WP service. In Soviet parlance the Iraqis had the export "monkey model" with rounds to match.
The T-54/55 is a very formidable tank up to the mid-late '60s. Have absolutely no doubt about that. To replicate the disastrous performance in the AIWs there are many ASL soft penalties that can be used as SSRs. Examples like:
6+1 Inexperienced Crews: 'nuff said.
Must be BU to fire: Usual doctrine/practice.
Must be BU to fire except if an AL in vehicle: Similar but allow for a few commanders who got their shit together.
I'm sure others can think of other handicaps. To my mind the correct approach is to use ASL's existing soft handicaps to reflect crew deficiencies without turning the T-54 into a fantasy Tonka Toy. Otherwise the player might get misleading lessons from history.
The MASL (Modern ASL) group proposed an intermediate AF step of 22 (between 18 and 26). While I consider that (18->22-26) to be a more rational stepping than the existing 18->26, it's far too late for that in ASL, unfortunately I fear.