Well well well. Welcome to the gr group
You need 3.5 Ratio diff to counter the excessive torque output.
It is easily available in crowns, mark x etc.
Remember just swapping the diff woll not hel Pat all you need to calculate the diff ratio before installation
Mark 2 has a bolt on fitting with crown diff mark x uses a different diff foundations plus it's axcels are hook fitted.
Mark x has commonly diff ratio of 4.1
And you need 3.5
Remember