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The shank of a 20R Toyota distributor fits nicely into the MGB block. The shaft must be shortened and the drive coupling from your Lucas distributor fitted to it. I used a strip of shim stock to take up the few thousandths difference in shaft diameter. I made a plate (out of an old aluminum road sign) that bolts to the block, has a large hole for the shank of the distributor and a 1-1/2"X5/16" carriage bolt that comes back out through the adjustment slot of the 20R distributor.
The only trouble with this set up is that the vacuum and centrifugal advance are in the WRONG direction. So pull the shaft with the centrifugal apparatus out and grind off the bottoms of the swaged on weight pins, and heat up the base plate and knock it off the rotor shaft. Punch out the pins, reverse the plate and braze or silver solder it all back. Try not to overheat the magnetic rotor. Next, scrap the vacuum pot and fit a bushing with a 3/16" rod that goes to the sensor plate (I heated it up, hammered it flat and drilled a hole through the flat for the pin).
I am delighted to report that this works great with the Toyota ignitor assembly and that the tachometer also works. It would greatly simplify the process to find a compatible distributor that runs in the right direction!