some persons sell front diff for 4x4 and say that is for a 3,9 ratio (i understand that is of a xr4 4x4 or similar car) but only with the front diff i have in my sapphire cosworth a 3,9 ratio? or the only way to have it is to change the two pinions and cut the chain of the transfer box?

and in that case one front and rear 3,9 ratios of 4x4 xr4 resist 300bhp of a cosworth?

thanks people