Paul,
Avoid the B&M shifters on the MT75 Cosworth 'box like the plague, they are horrible for this and cause issues with the syncros etc (as well as vibrating like a bastard).

What you do want to do is speak to Martin (as Spadge has mentioned), and ask for his solid bush kit for the 'box. These are some metal bushes that remove the "slop" in the gear-change, but don't cause problems with the syncros - job done .