I'm with Dave on this. If you have a decent B3 leave it as a vintage racer.
They're nice to drive still, but simply not fast enough or tough enough to race amongst modern cars.
Also, very little fits from a b4 to a b3. Gearbox wise nothing but the topshaft to fit a more modern slipper BTW (different gear ratio, so both idler and diff are different size).
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