71 LA 340 timings moved!! how?? looking for help

Well, the cussing part of the language is identical on both sides of the pond LOL

Have you rechecked the timing at idle after of this? The prior suggestion that the timing was hung up at a highly advanced position may be the answer. You have moved it back, now it may have ''un-hung-up", and may have jumped back to the old idle piston in the distributor and it is far too retarded.

And does it advance smoothly as you increase revs, with the vacuum advance line plugged? You're gonna have to put the timing light on it a lot to figure this out, IMHO.

No way to guess on the noise from under the valve cover... pull it and examine. Take pix and post of any thing suspicious.