Brake drum question

Naw, those are junk, send em to me . I'll trade you some other ones I got kicking around. Those huge flanges just get in the way of everything....Hah. Kidding!

Do not modify those drums. They are getting harder to find all the time. Those "huge" flanges serve two purposes. #1) they dissipate heat faster,than regular drums, and 2) they help keep the drum from bell-mouthing. Those things,on the back of your A-body, for street and mild performance driving,are probably the best thing you could have back there.
But if you really want to get rid of them, put me first in line.

Oh, in answer to the cutting off, of that centering ring; DONT do it.
Better it would be to machine the adapters. But best would be to get proper-fitting wheels.
That centering ring performs a special function. It may never get used on your car. But in hard running/braking that drum may get very,very,hot.Thats its function; to convert forward motion into heat. When it gets hot, it expands. A lot. When it cools it contracts back to its former size. That centering ring makes sure that when it does its growing and shrinking, that it stays centered on the axle. As long as its centered, it stops smoothly. If the drum were to shrink back off-center then on the next brake application, there would be a terrible vibration created inside that drum, as the off-center drum forced the shoes into an ever-changing eliptical path. If you shave it, you would be compromising its ability to do its job.It fits into the cavity of a stock wheel, because of the way that wheel is formed. If you look at period aluminum wheels, you will see that they were machined for that"ridge".If you look at the back of your cast adapters, they should be too, or at least have plenty of clearance.
BTW; They used to make adapters, SEMA approved. Depending on the class, we used to be able to run those in dragracing.