Looky what I found in the neck of my 86 G/S.
I could hear loose stuff in tank (see thick piece of red lining in pic) and it was driving me crazy, trying to get it out with flap in place. It also seemed like the deep neck would easily decrease tank capacity by a fair amount. Google found me this: http://www.airheads.org/content/view/288/98/
It occured to me that what I was about to do was really stupid and i'd probably ruin my nice tank
In fact it was fairly easy and not that big a deal. A half doz dremel cutoff wheels later and I had a nice smooth, straight neck opening. Cool