I'm not familiar with the 40's vintage trucks, mine is a '50 and a 3600 (3/4 ton) so I'm sure the information that I have may not be directly applicable to your situation.
However, if you have the bell housing mounting points on the riveted in cross member I don't think you want a 2nd transmission mounting point. My understanding is that the transmissions are fully supported by the bell housing and adding another transmission mount at the rear of the transmission can create a problem. However, the bolt in cross member your seeking may provide frame structure and not a transmission support. If it is behind or below the transmission making it removeable would make sense to be able to move the transmission back for removal.
Perhaps someone with more direct knowledge of your vintage truck can provide better insight.
