New 300sdl, rebuilt trans slipping in first gear only

[bigpanda16]

Not sure if this doesn’t belong in the tech help since it’s a gasser trans...
Way back I found a rebuilt trans, could be sun valley, could be else. The car was an m103 with a tired shortblock, rebuilt head and this transmission. All parts had the star, including the trans filter.
Point is, this transmission(722.358/368, I forget) I have put into my wife’s new 300sdl which I saved from the crusher. It slips in first gear only. And only a majority of the time. When warmed up, sometimes acts normal.
To clears things up, it has a super strong reverse and goes into it right away.
The one variable(I did have to swap the tail housings due to speedometer variations) is that the Bowden cable has a vaccum line coming off of it.
I’ve searched but couldn’t find a definitive answer.

Unfortunately it could be a mechanical problem. Once I can milk it into 2nd, it holds firm through 3rd or 4th gear it holds power no issues. It is a surprisingly strong om603, really ripped up to 80mph

I don’t know if the vacuum line needs to see full vacuum or modulated vaccum like the other side for shift harshness.

I’ll give it another go tomorrow, hoping someone else has run across this
Thanks