Absolutely! If your plan calls for pins in the back rail to guide the keys, drill them while the keyboard and keyframe are still attached.

When I built my first harpsichord thirty years ago, I thought it would be my "one and only". As a result, I decided to make the arcades using forstner bits. They came out so well, that I stayed with them for all these years rather than have a custom bit made.

As for gluing the bridge to the soundboard, the traditional method is to use go-bars. Lacking a go-bar deck, you can nail the bridge down from the top, through the soundboard, and into your work table (as long as you don't mind holes in your work table). Place the completed bridge on your plan and make a pencil mark for each nail that will not coincide with a bridge pin. Cut enough small rectangular pieces of 1/8" thick scrap, like cut-offs from your soundboard, drill a hole in the center of each big enough to let the nail slip through, put the nail through the hole and use it to clamp your bridge to the soundboard/table. Don't forget to pre-drill a hole all the way through the bridge at each pencil mark, but not into the table below. After the glue has cured, break off the scrap wooden pads which will expose the nail head about 1/8" above the top surface of the bridge. That will allow you to grab the nail head with a pliers or vice grip and pull it out. After they're removed, glue a round toothpick into each hole and sand flush. The term "nails" is a misnomer in this case - they're really wire nails, about 1/16" in diameter and long enough so that they bury into the table at least 1/4". You can buy them at any big box store.

This picture should clarify my ramblings a bit.

Ernie