First of all. Thankyou everyone for replying so promptly with solid advice and thought.
Second, it's a ground up rebuild. Every piece was removed and cleaned from top to bottom.
Mainly top end modification. I got a cam grind, head shaved, new carb, springs, electronic ignition, etc.
I've done all the assembly myself, making it slightly more irksome as it ran well before I rebuilt it.
After a painful night of taking the engine back out of the car and quick disassembly, culminating in giving up at 2am and restarting this morning, it seems that it was indeed the conrod caps being replaced in the wrong orientation. Similarly, I also put them on in exactly the same way I took them off, evidently the wrong way! The engine has shown signs of prior rebuild being done poorly, but I didn't think extended this far.
Unusually, the piston are obviously not original as they lack any of the marking described here or in the yellow book. Just some numbers and letters which don't seem to correlate. SO I just changed them in faith that would be the solution knowing that bearings were in good condition, and the camshaft was properly installed, so it wasn't valves or anything similar causing interference.
When I built it the first time I could turn it over via the crank bolt, but with effort. Now upon cap reorientation, the whole assembly rotates much more freely.
I've reinstalled the engine and the starter seems to be doing its job well without the spark plugs in, until the battery went flat anyway.
However, I am optimistic this was the solution. And on the bright side, I know I won't let it happen again.