so my nak deck does a very nice job playing back my old analog masters. however, i think the pitch is slightly off compared to the speed of the old D6C i used to make the recordings. i tried the speed correction tool in audacity and it doesn't seem stable/consistent when i speed correct a segment. is there a better (free) method to fix the slight speed issue with these transfers?
Do not use Audacity for speed correction. It is terrible for that (and many higher end processes). Fortunately as a taper you have ears to notice (unlike some who use it and don't).
As you found it is virtually certain that any two analog tape recorders will run at different speeds (creating a pitch variance when a tape is played back elsewhere).
Assuming you had stable speed end to end on the master (and assuming they all recorded at the same running speed over time) you should be able to find a consistent offset. If so you can apply that to each recording (either cassette side by cassette side, or to the entire edited result, if consistent). The best thing to use is a solid audio editor. Audition (or Cool Edit before it) is very good for this (using the Pitch Correction function in semi-tone scale, not %, and not Pitch Bender). You can save your pre-set there and keep using it on all of them if there is a common offset. Other professional editing tools will also have that function. It all depends on what you use (just don't use Audacity).