I quote from Fisher Body Service Manual 1937, which covers the Buick as well as other GM brands.
"The dash panel is a steel stamping electrically spot welded to the cowl upper panel and the cowl side panels...a dash to sill brace is riveted through the dash to the chassis frame brace. The same rivets anchor both of these braces to the dash. These braces form an anchorage for attaching the body front end to the chassis frame.
The dash to chassis frame brace...is used on some style bodies, while a differently formed brace is required for other style bodies. All serve a similar purpose, being riveted to the dash and dash to sill brace in approximately the same manner."
It's a little hard to follow some of the references (braces, etc) but the short answer is that it is welded. The missing words denoted by ... are references to drawings in the book.