Hi There
The box with the little (cool) lever is a GKT 134 used up to '47.You can order a gasket set from http://www.classictruck.com/category/s?keyword=gkt+134+gasket

A bit of info on the later gearbox....
The '48 onwards used a Muncie SM420 which bolts up to bell housings with the standard Muncie Chevy bolt pattern. To put one behind a 350 you need an early '56 Chevy bell housing with the open bottom to access the lower threaded bolt holes in the transmission.

Hope that helps