So I have a 1968 coupe with a 390 paired to a C6 tranny and after years of resto I finally got it on the road yesterday only to find the the transmission will not shift out of what seems to be first (maybe second, im not sure). Regardless im in the same gear from a standstill all the way up to 50 or so which was the fastest I dared go. I then tried shifting manually from 1-2-D however from 1-2 absolutely nothing happened and the 2-D there was a lurch when it shifted gear but it still felt like I was in the same gear as before, the one it doeant't want to shift out of.

Now obviously I know very little about transmissions seeing as when I came home and did some research I found that I had no vacuum line attached to my modulator (which I didn't realize existed until now) and so I have a new modulator and line in the mail. So this brings me to my two questions: should having the modulator properly attached fix the issues in D and/or manually shifting 1-2-D, and if not what are some things I could check for??

Any and all help is appreciated
Thanks,
Jack