1982 240d transmission

[Sillyaz]

Hi there. I'm brand new to this forum, but have been watching for a while. Great place for valuable info. My question is... what automatic transmission is compatible with my 82 240d? Will an automatic from a 300(sd, d, etc.) bolt up? I have the manual set up complete with all the hardware, and have read many a post on the advantages of this application. I just don't know where to start. Would it be easier to drop the old auto, and replace it with another? Thanks in advance.