I know ONE man who has successfully shortened an SKS and made it work. He is a retired professional engineer who has been working on firearms designs since 1945.
But you asked a question, so I will predict a few of the results.
1. you would have nowhere for the bayonet to lock up onto
2. the gas-operated action of the SKS depends on a specific PULSE and DWELL of that pulse for sufficient power to operate the action. If you upset this more than a very small bit, you are going to REALLY affect reliability, quite possibly to the point that you will need to develop entirely new loads for your existing casings.
3.NOISE LEVEL and MUZZLE FLASH would increase dramatically. You then get to add a muzzle brake and increase it even MORE. Fire more than a few magazines and you end up a deaf old fart like me, having to SHOUT to make myself heard....
4. if you lop more than 2 inches, you get 5 years of FREE room and board in a secure environment, with FREE ### thrown in. You might not like that.... and the food is lousy. The Charge under the Criminal Code is "manufacturing a Prohibited Weapon".
5. you destroy what accuracy the thing has. That Industrial Hard Chrome liner in the barrel is the utter devil to cut evenly (wrecks hacksaws in no time flat) and an imPERFECT cutting will destabilise each and every bullet which leaves that barrel.
That's for starts.
I am sure that some of the guys who have worked with the Simonov Self-loading Carbine more than I have will chime in with their own reasons.
For what it was designed for (deliberate shooting at man-sized targets out to 300 metres), the SKS is just about perfect as it is. Why mess with a Good Thing?