Thank you all for the really great responses. I did check with my local gunsmith about drilling and tapping the receiver so I could install a standard Williams FP 99S receiver sight. He said it would cost $60 to drill and tap, but on some of the 99s they had to anneal the back of the receiver because it was too hard and that would cost an additional $20 if they had to do it. Luckily, I did find a Redfield tang-mount receiver sight on Auction Arms. Not cheap, but less money than the drill and tap program and no chance of butchering up my 99. I think I prefer it to the Marbles tang sight. Time will tell. If I don't like it, I can always sell it, probably for what I've got in it or more - that's the nice thing about old classic receiver sights. Thank goodness for the internet. I would never have found one without it. Surprisingly, I found about four different ones on eBay, Auction Arms and Gunbroker. Some of them went for as much as $160, but at least they are out there.
Thanks again for your responses.