What don't you understand about correcting the turret?
You have some good choices for barrels...it depends on how much you want to spend.
For the .50 Cal....
For the 7.62 mm (C5 GPMG)....
But, this is not just about changing barrels....if you want to use the 7.62 mm C5 GPMG barrel you need to back date the turret and the turret sight (look at Ying's build).
If you want to build a 1994 or later Grizzly you will need to replace the 7.62 mm GPMG with a C6 barrel of your choice....the kit has the correct gunner's sight for this version.
For that barrel look for any C6, MAG 58, or M240 machine gun for the barrel.
For current turret choices you can...
1. Correct the kit turret...see Ying's build.
2. Buy the Best Value Models Grizzly AVGP conversion and use the turret.
3. Buy the Best Value Models M113A3 TLAV conversion and back date the turret to a Grizzly uparmoured turret....but then you will have to add armour to the entire hull.
4. Be patient as a correct resin Grizzly turret might come along.