The investigation went further and here are the results.
Lack of oil:
I rechecked all the the oil passages and they are clear. Used an oil hand pump and just pumped oil into the passage that feeds the rear bush and clear oil came straight out of the holes on the plain bearing. By covering one hole with my finger, oil then flowed out of the holes on the front bearing. So all the passages are clear as well as the grooves on the outside of the bearings. That means the oil supply was there. The only thing that maybe was not ideal are the positioning of the holes. According to one of my mates who is an engine designer and also restores his own classic cars, the ideal positioning of the holes for this type of bush supporting a high load shaft, and the main load being straight up and down as the pistons fire, would be at 1 & 7 o'clock or 11 & 5. Being aligned with the oil passages in the crankcase, the oil flow will go mainly straight to the bearing through the first hole, and maybe not have enough back pressure to flow around the groove sufficiently to the second hole and much less to the second bearing on the front crank. As I had my holes aligned, the front plain bearing would get much less oil than the rear one, and even less out of its second hole which was positioned at 3 o'clock. However he still thinks there should have been enough lubrication even if not the maximum one could get out of the system.
The seized bush shows rubbing/friction marks on both inside and outside edges. Inside the crank was pushing against it, and outside the thrust washer. Compared to the rear bearing, which is nice and yellow, these edges of the front bush are white and show clear signs of wear. Friction marks are also to be found on the crank shoulder and on the inner side of the thrust washer. Looking at the damage to the white metal on the bearing surface, it is only damaged and broken away in the first few mm going in from the edges. the centre part is fine and likely didn't seize. So it seems to be that the friction against the edges was so much, that it developed enough heat to melt the babbit in the area just in from the edges. The bush was squeezed in between the outside thrust washer and the crank shoulder on the inside. So I had absolutely no end play on the front shaft! I know on the front crank I had almost no play and didn't add any shims.
The big question is why, and what did I do wrong during assembly. I read and re-read everything on how to set the end play, but probably didn't do it right or misunderstood the whole thing, so here my flow of new questions
Please confirm or correct the following:
The axial movement of the cranks is controlled or limited thus:
1. towards the drive side the movement is limited by the roller bearing and the bearing itself by its retaining circlip.
2. towards the timing side the crank would move until the crank shoulder would hit the edge of the plain bearing, which should not happen and a certain clearance is needed.
3. the correct assembly on the outside timing side is - the thick thrust washer, then the necessary shims to set the proper run-out, then the nut. For the rear crank the half-time sprocket comes between the shims and the nut.
4. The shims can only reduce the run-out, not increase it, since adding shims just causes the crank to be pulled out when tightening the nut.
On my engine, there seems to be almost no run-out or end play on the cranks, and I didn't use any shims. So much so that when I tightened to nut on the front shaft, I just squeezed the plain bearing between the crank shoulder and the thrust washer leading to disaster.
I must be missing something or getting something backwards and completely botching things up….
So, very hopeful one or more of you chaps spots my mistake and set me right. And then call me an idiot not worthy of touching such an engine….