I also have a 6t B with a 3 synchro and a 78B with a 4synchro with overdrive.
To convert you would have to replace the backing plate, starter, flywheel and
transmission. You also would have to modify the tunnel to fit the 4 synchro.
The three synchro is very easy to rebuild. It is essentially the same trans
as in the MGA. A friend of mine can remove his MGA engine and trans, rebuild
the trans and install it in less than 4 hours.
True, the earlier 3 synchros had a smaller diameter layshaft, but it can be
converted if desired...Me, I'd keep the 3 synchro.
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