40-198 for a 1-5/8" wide door

Hello, Lew. The door thickness doesn't really have any bearing on whether or not the 40-198 will work for you. What's more important are its backset and thickness, and to a lesser extent its other dimensions. 

I recommend carefully reviewing the measurements we show for this product. As long as they all match then it will replace your old mortise.

Alternate image for 40-198

Side view of 40-198

You're very welcome! Please let me know if you had any other questions and I would be happy to assist.