You could run the left right mix to your PC, worst thing you would need is a RCA to mini jack adapter ( pending your PCs inputs ).
http://www.cfbsoftware.com/lprecorder/lprecorder.htm Can be used for the line in source on your pc...
http://www.kreatives.org/kristal/ Can be used with the LPrecorder line in source to multitrack ( stack tracks ), or record left right mixes without the LPrecorders 4min time limit.
Outside of that its nice to see someone using a tape deck "
If you clean and d-mag the heads on the deck you can get fine results, if the tape deck has some noise reduction even better.