Will a 1991 chev engine drop into a 1999 youkon if not what are the differences ?

 

Mattymott Welcome to the forum...
Both years used the 350 or 5.7 engine so its likely the motor mounts are in the correct place but there is a big difference in the electronic controls...
I'd say the mounts are going to be the biggest concern, but if you can weld that can be overcome too...

 

On the whole, I don't think you'll have any insurmountable problems.
I just put a 1999 Tahoe 350 in my 1996 K2500 GMC Suburban. Everything bolted up.
GMC/Chevy has always had a good reputation for parts interchangeability over the years.
Good luck.

 

same block but the fuel injection system is different, youd have to swap the heads along with the intake /fuel injection. i dont think you can get the new computer to run the old engine, the 99 system is far advanced compared to the 91