Ken,
I'm not specifically aware that the timing was changed at the point of going to a single row chain and sprockets.
Why not get the adjustable dual row cam sprocket and put the timing just exactly where you want it? Advanced Performance Technology is coming out with a camshaft just for the "blower" MGB, and certainly they have the knowledge to tell you where to time it for your specific needs. between these two, you can surely optimise what you have
Motorbill
From Lola to Land Rover, If it's British and has wheels, it's likely I've bloodied me knuckles thereupon