I have a 1970 F-100 with a 390 & c6 tranny. If I drop it into first and stomp it I can't get it out of first unless I stop and then it is still hard to get out. If i put it in first and go easy on it and shift to second it shifts fine. A guy I work with who has the same year truck has the same problem. Is there some way to remedy this? I thought it might be linkage but I wd40'd the heck out of it and checked its position in first but it doesn't seem to bind. I really need to have it so I can shift manually because 3500rpm automatic shift points just won't cut it after the motor gets rebuilt to around or purt near as some say 400+ HP. My motor mounts are getting pretty old but they aren't broken. This is my only guess to where the problem lies.

 

If your going to rebuild the engine why not take advantage of the oppurtunity and get the transmission rebuilt with a good shift kit while you are at it? but I'd check the vacuum lines too

 

The tranny is getting rebuilt about 5 days after the engine is done. It will have a shift kit and high performance clutch packs but I don't know if that will fix my problem

 

I know the '74 LTD my grandfather had you could floor it and hold it it would not shift out. My dad did a test run after changing carbs on the 460 it hadn't shifted out of 2 gear at 90 when it blew a freeze plug. Sorry couldn't really help.