[Mike Taylor 3601]

I'd say slider/dog problem,but a few more things has bellhousing been checked for alignment? My brother had one out he told me drive it maybe I can't drive,I can't remember if it was 3 or 4 or both but it was like you hit a wall with shifter would not go wide open or even the shameful Granny shift. BTW the term Granny Shift offended my Granny's,God rest their souls. His bell was like .050'' out and was way out of parallel we had to find guy with blanchard grinder big enough to true it up and we centered it.After that would fall in gear,still will need to service sliders/dogs once they are wore they will actually push apart instead of engaging.
Also make sure pilot bearing is good input out of line from bell or loose pilot bearing will let input move and keep from shifting.usaully misalignment high gear will be the worst but if you can't get high it's hard to tell.
You need to service trans and check all the things that have been mentioned in all these posts so you got it right and will not problems shortly.
If you have problem with pilot bearing the only type I have seen that will last is bearing w/sleeve on od that fits in the big hole where,please excuse my language,but i'm just gonna say it ''torque converter'' centers,well anyway on chevrolet.Jerico sells them
I had slider getting bad in Nash one time if you did'nt hold in would jump out of third when jumped out kept pushing when it bumped high side would go back in and most people never knew anything happened.
Mike Taylor 3601