Cosworth 2WD diff is wider than the one you have fitted and will not fit your beam without modifications.

Have a look at the tag on the diff you have now and use the link I posted to find out what the ratio is, if you're struggling with low end torque you'll need a lower ratio diff to help pull away.

There's not much point changing it if you end up with a similar ratio, I'm surprised even a tuned Essex V6 has a lack of low down torque though!