I am not super familiar with older vintage stuff (I stopped riding from '88 to 2014), but this picture makes it look like that tab is broken off. I understand what you are saying about bending it, but that end looks jagged to me, like a horizontal piece is missing. Bending might just draw the inside plate in, and cause it to rub the pin link plate. You could trade one issue for another. Replacement RD might be the best avenue.

As to chain width, it looks to me like inside width of the roller unit hasn't changed from 6 speed on up. Outside widths have changed significantly, but I originally asked because I thought narrower rollers might ride up on your idler sprockets. That does not appear to be the case.

thanks RollCNY, I think it is just well worn from almost 30 years of use as the photo shows :-) I agree it may be tricky to bend without hurting something but i'll give it a try. I have 2 other bikes to ride if this one is down for a few days.