That you would never believe

Are these sentences correct:

1) When writing his letter, he made such mistakes that nobody could understand his meaning,

2 When writing his letter, he made such mistakes as you would never believe.

3) When writing his letter, he made such mistakes that you would never believe.

4) When writing his letter, he made such mistakes that you would never believe them.

5) When writing his letter, he made such mistakes that you would never believe he made them.

Gratefully,

Navi.