Even a 1950s era transmission will still be very hard to force into a gear if the speed difference is too great. Likewise you'll have a very difficult time shifting into reverse or 1st at speed. That's just how syncros work. You might be able to overcome it by double clutching but it would require conscious effort. Even just shifting to reverse while rolling is difficult.
Still possible though. Years ago a friend and I pulled a manual transmission in a junk yard. He got it home, installed it, and then discovered reverse was completely stripped.
