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.
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.