Today I was in the Stang. I noticed it seems to delay shift from 1st to 2nd and when it does shift it shifts kind of hard. 2nd to 3rd is no problem.

I think my transmission fluid might be a bit low, could that cause the hard shifting between 1st / 2nd?

 

Could be. Might also check your kickdown cable and vacuum modulator. Better find out why your transmission fluid is low also.

 

Yeah check what he said.

 

Be sure to check it running in park with the engine up to temp to get a correct reading. May be time for a flush/refill.
Also there are 2 rubber vacuum hoses on the auto's, one 90* elbow at the trans. mod. (rear of trans) and one at the rear of the intake. There is a metal line that connects them, follow it to find both. Most times these hoses are never replaced in 40+yrs. so it wouldn't hurt to change them, be sure to use a 90* formed hose on the rear or it will kink.
As mentioned the trans. modulator could be at fault also, there are different ones in the same year, they are usually marked with a paint stripe (red, yellow, etc.) so be sure to clean yours off to see the color before replacing, the different colors have different shift levels. Also find out where you're loosing fluid if you are as mentioned.
Jon

 

If you can't find out where the fluid is going, there is a good chance that the vacuum modulator could have an internal leak and the vacuum is pulling the fluid up into the intake and burning it in the combustion chamber. This would also account for your shifting problem.