Front wheel drive gearboxes

I cicuit race a ford laser tx3i (can be seen on my web page, www.sss.net.nz) It runs a mazda bp1800 dohc turbo engine making 400hp @22 psi.The class i race in only allows front wheel drive.I run the mazda 2wd gearbox with the 4wd internals and a locked diff.The gearbox breaks after about 3 meetings(stripped teeth etc).I want to replace the gearbox with a stronger 5 or 6 spd box.I can fabricate adaptors etc.Does anybody have any suggestions!!!!!