The rule is pretty simple, 30mm penetration is 1 AP. If you have a round piercing 300mm, it's 10 AP.
The M60A1 has 11AP and that's using either the M392A2 or M728 which are around 225mm-250mm. M60A3 and M1 have 13 AP and they both use the M735 which is 300mm.
The armour and AP on the early Merkavas seem to be a little too high in my opinion, especially when considering when they were introduced.
Little?.. They are overmodelled as fuck, and not only early ones.
The Merkava I uses the M-111 with 320mm pen - Merkava II uses M-418 which has 380mm, so the AP values are roughly correct. I'm more concerned by 18 FAV and the grenade launcher on the IIA.