Mono is equivalent to 1.0.
Dolby Stereo, aka Dolby Surround, is a 2-channel format with surround information matrixed into the signal; on playback it is decoded into 4 channels - left, centre, right and surround. Dolby Surround tracks are labeled 2.0 surround.
The Dolby 6-track had 6 discrete channels - left, right, centre, surround and 2 low frequency effects channels. This would be 4.2, but that configuration doesn't exist in home theatre setups. However, it could conceivably used as a source to remix to 5.1.
The soundtrack was also remastered twice for home video, in 1985 and 1993. These two mixes were also in Dolby 2.0 Surround.
Normally in 2.0 surround, there is a flag in the Dolby Digital stream that activates Dolby Prologic decoding on the receiver/decoder. So you should automatically get surround sound, if you have the equipment. Dolby Surround was the format used on the majority of movies in the 80s and early 90s - it was good then and it's still good now.