Same as OTL, india starts a new front in punjab
and pak army has to pull out forces to reinforce its border in sialkot and lahore sectors
the biggest issue here again is beans and bullets
pak army of 65 very dependent on all kinds of foreign supplies and probably could not sustain more than a month of hostilities
esp if karachi is blockaded by indian navy
Operation Grand Slam was a pre War general staff plan. It was predicated upon trying to pre-empt Indian attack on the Akhnoor-Sialkot axis.
It was never supposed to be a strategic offensive plan.
In OTL it was used since it was the only one which could be implemented in the short period available.
The fact it nearly succeeded is due to the tenacity of Pakistani troops and the frankly abysmal performane of the Indian General staff.
It would have been like the dog which caught the car.
They were on fumes in OTL. They had to raid the reservists pool quite deep, and by that I mean they were recalling WW2 veterans as garrison troops to free up men for fighting formations.