My TH400 with a forward manual valve body will not shift into first gear until the transmission is warmed up, when it is warmed up 1st gear works fine. I checked the linkage while shifting through the gears and it is going into the first gear position. Any ideas???

How old is the trans? Possible clutch piston seal issue.

How old is the trans? Possible clutch piston seal issue.

The transmission had the same problem prior to a rebuild and it continues to have the problem, trans shop stated that they could not find any problems with the tranny.

Well, if it was just rebuilt, sounds like one of 3 things. Internal leak in center support, low area on case near center support feed, or thin center support without snap ring type shim below it.
Also, if the 1-2 accumalator (under rear servo) is still used with your shift kit that could be a problem.

The forward clutches are fed via the front pump. Make sure the sealing rings/surface they ride in on the drum are okay there as well.;)

I'm not sure what you're saying. Does it not upshift when it's cold or does it not move when you put it in low gear when it's cold?
My answer refers to shifting, Oldani's refers to moving.

First gear and second gear feel the exact same when cold. When warmed up first gear works fine. The car does move and the tranny does not slip.

When it's still warm and running okay, will it downshift to low gear as you come to a stop?

It is a full manual valve body, when it is warm and I shift into first at a stop light it works fine. Just doesn't cold.

Look at the line pressure in low gear when cold and see what happens as it warms up. I feel if it's heat-related with a full manual valve body you'll find it to be a forward clutch piston seal issue as the cause. Something is not sealing up when cold but is fine after warming up. Usually this is found only if the seals are old and hardened up though. Unless there is something very strange going elsewhere. The full manual TH valve bodies are usually foolproof and never have issues since there's not much left in them to go wrong.

Thanks, I will give it a try.