[RICK LAKE]

mrmustang Bill I would start with putting the car in the air and checking linkage from under it. Make sure the linkage is clearing the tunnel. Look for marks on the tunnel for possible rubs or hitting.
Motor mounts and this is a biggie, take the hood off the car and go for a spin through the gears, How much movement is there of the motor going up and down the trans gears? In the old days you needs either a chain with a turn buckle or a torque strap to the block and frame to stop twist. The clutch linkage would bind and cause shifting problems. I am going with the fact the clutch is alright. But if the linkage is binding---. My fork on my car was hitting the tunnel under load. I have modified it to clear. ERA just came out with a new fork for the clearance and the correct length and angle for their cars. They have one for GM input shafts. Less movement of the TOB on the shaft and stays centered better.
I don't think it is blocking rings or 2 bent shift forks. The shift goes from 1 fork to the other main fork. It is possible that the pellet in the internal of the trans may not let the fork return to a neutral postion and is stopping the shift to protect you from putting the trans in 2 gears. I would change fluids to a lite one and retest problem.
The last thing is to change fluids to lite one. Used to be heavier weights was better and now ATF is the in thing.
I am thinking motor twist from weak motor mount of rubber. On the FE motors I have, had to bolt the 2 halfs togeather or get the same problem the motor jumping out of the car. The rubber mounts stop vibrations of the car, solid mount will stop all movement. If you go with straps or chain to the motor in most cases you will end up with the top going into the head. I would use a stud of grade 5 or 8 for the upper mount. If you can't find something than go get a long bolt and thread it and cut to fit. Washers and nyloc nuts too. I know it's a pain but put the strap or chain behind the motor at the bell housing. If you mount the chain in the front of the head, the motor will twist at an angle and wear out the mounts if rubber and bind the linkage.Saw a throttle linkage do that one day and wasted a $35,000 cammer motor. The guy had no limiter in the MSD to protect the motor, 8,900 rpm and bang.
Good Luck Bill, Happy New year. Rick L.