Don't drill and tap the tang if the serial numbers are on the tang. Buy a tang sight base adapter. If you drill and tap through the serial numbers, in the federal laws you are defacing the numbers. Not only will the rifle be worthless but you can be arrested and criminally charge and after conviction you may not ever pass a back ground check again.
I'm certainly no expert, but I thought you needed at least one hole drilled for the adapter.
MARLIN MODELS 336, 30, 1895, 1894
Installation of the Tang Sight on Marlin Lever Action Rifles must be precise so
that the Tang Sight is perfectly centered in the cutout in the rear of the bolt
when the bolt is in the full open position.
It is necessary to use the supplied adapter block so that the serial number on the
upper tang is not damaged and the height is correct. Three screws are supplied
with this model.
All Marlin lever actions must be drilled and tapped for the
middle adapter screw. Remove the lower stock screw, insert the new longer stock
screw to align the adapter on the tang. Mark the location of the center adapter
screw on the tang. It should be drilled and tapped for one of the short
10 - 32 screws supplied. Once drilled and tapped, align the adapter with the long
rear stock screw. Tighten the adapter in place with the center adapter screw.
Remove the stock screw used for alignment. Place the tang sight on top of the
adapter, insert the stock screw in place and tighten slightly. Insert the front tang
screw. Tighten both the front and rear tang screws evenly.