I had same problem recently with stops on now-mine ?1984? Ross Signature 290s that I am converting to bar-ends. Ended up dremeling the hell out of those new stops

Good news is that aluminum is not that difficult to remove, and end result looks good if well done. Bad news is that there is quite a bit of material to remove and there are plenty of chances to screw-up.

In addition to cutting obvious slot for the 'key-shaped' part of the boss, I had to make stop 'deeper' by about 1.5mm in order to put it flush against the frame.

First stop turned up to be just OK, since I cut the slot for 'key' under wrong angle, and ended up widening it, creating a gap. Second stop looks perfect.

Now, I wish I had proper 4.5mm bolts... for now I am using those HUGE and ugly bolts from original downtube shifters. Looks funny but I do not have a heart to re-tap the bosses to 5mm. Maybe, later, one day.