Odd noise

[Charles P]

To me, it sounds half engine speed, which is why I suspect it’s camshaftey.  I’d be surprised if the amount of rod clearance you have is causing the problem, but if it is it should be rectifiable by relieving the rod or edge of the cam lobe. I’ll measure some cams tomorrow to get an idea of what the differences are.

Thanks Alan.

First thing tomorrow I'll whip the manifolds and valve cover off (not so easy on a blown engine).
If Ian's theory is sound my very carefully set valve clearances will be up the spout as a result of the shoulder issue. 
Following your comments about clearances with Barlow rods I did spend a vast amount of time aligning the crank, rods and cam to try and avoid a collision problem. It's possible that in aligning it that way I've overlooked the follower clearances.

Charles