A very nice old TBG Co gun there Friend, and what a thrill it would be to take a whitetail using that piece. These guys are correct, your best alternative is to raise the height of the front sight; and as all the examples I've seen are dovetailed into the top rib, finding a replacement to work shouldn't be too difficult a challenge. I would strongly encourage not using a tang sight, as doing so would require drilling after-market holes into the top tang; and if you retain the sliding shotgun safety along with the new sight, you'd be forced to notch the comb of the stock to get enough set-back for the sight base. I know of one very nice and rare little 20-bore Hollenbeck to which such a tang sight was added; and not only did the installer screw-up the top tang and stock, it looks completely out of place on the gun. On the other hand, I've seen one example of a Hollenbeck drilling that featured a factory tang sight. That sight was inserted into a very neatly formed/sculpted hole in the stock just ahead of the comb. It was not attached to the gun, but was a separate piece designed to inserted for use when needed; with the result that it did not conflict with the flowing lines of the gun when not in place.
I don't own a Hollenbeck drilling (yet), but find these guns most intriguing. One of our members owns drilling number 7; and some of its features are a skeleton butt, tubular cartridge trap located in the checkered butt end of its stack, sculpted barrel lumps very similar to those found on highest grade Lefever guns, and a very long trigger guard bow that also covers the rifle barrel cocking lever. I do however own a rare Hollenbeck Gun Company double gun; and although this old gal saw lots of use and needs TLC, she's all there, has excellent 30" bores, no after-market alterations, and remains in good shooting condition. This gun is #28, is a 12-gauge, is not grade marked, is English stocked, features almost 100% engraving coverage with game scenes (pointing dogs or either side, and covey scene on trigger guard bow); and weighs in a 7 lbs./6 oz. It also has a few other neat features such as finely checkered triggers, "dimples" at end the muzzle ends of its barrel ribs very similar to higher grade LC's, and highly polished (plated?) lock work.

I wish you good hunting, and please post a pic of this gun and your whitetail deer bagged with same.