Transmission Not Shifting

[Kisner]

Now that the 68 stang is finally restored, I took it out for its first test drive in over 8 years yesterday. I've mentioned this before but the 302 and C-4 automatic transmission were completely rebuilt in 94, had about 50 or so miles put on them and then it sat all these years. Was cranked about every weekend, oil changed regularly and I personally changed the transmission fluid 4 times over the past 2 years (while attempting to stop the pan from leaking). Drove it about 3 miles down the road, but it took forever to shift to 2nd and never shilfted to 3d. On the way home, it shifted about normal to 2nd and finally did shift to 3d but took a long time. Anyone know what I should check first? Does it possibly just need to run several times or could all my seals and everything else have hardened or gone bad? Everything is new, lines, modulator, etc, and hooked up properly. I know my Edelbrock 600 cfm carberator is not adjusted properly (idle sounds fine but the mixture screws are bottomed out clockwise) so could that be affecting the vacuum? The first I'm going to do is check the fluid as I know I've lost some due to the transmission pan leak. Hopefully, that's my problem. I'm also taking the cheap piece of crap $20 chrome pan off soon and putting on an aluminum one with rubber gasket to hopefully stop the leak. Yes, everything else has already been checked and it's only coming from the pan. I'm clueless as usual. Thanks for the help.