As those gents said, FD gearbox and diff are actually quite strong. But mountings usually cause them to fail.
Torque that your 20B makes, will probably blow up that T56 too, so I reckon that best optio for you is to use those cheap 5 speed Renny boxes. However, you will have to change clutch type.
FD diff can take lots of beating. That wheelhop tels that your bushings are gone basically. So change all of them. Get diff brace/cradle (dunno correct word for it), and pinion snubbers. Also, gearbox and PPF should be bolted together proberly. Any movement can make them fail, especially PPF.
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