Above 350 BHP increase the chances of breaking the transmission, the box is meant to be the weak link above that & the clutch would also need doing. Your £6K will go quite quick if you need an uprated box & clutch.

No point having a lot of power & not being able to use it because the gearbox breaks everytime you give it some beans.

If it was my money I'd go to MAD in Maidenhead or MSD in Blackpool, Stu has a good rep & he's got some good plans for the Small Turbo Escort. He's working on a 350BHP conversion for the ST Escort & I'm sure I read that Wils car is up & running again so they can finish the development work (Hopefully).