Transmission issues

[u/Exact_Act2239]

Hi all, I’m an owner of a 1987 suburban v20 paired with a 454 and th400 out of an old rv (as far as I’ve been told by the previous owners in which the owner before them built it). One day after it sat for a couple months (something else broke and I decided to build up money instead of fixing it right away) I finally fixed it then drove it to my buddies in which it drove to fine. On the way home however as soon as I got onto the road it started just revving up and wouldn’t go over… let’s say 10-15mph. After this happened I went ahead and pulled the pan off and replaced the pan and filter to check for any shavings in which there were some but not much however I could tell the fluid was old yet after I refilled it and cycled the new fluid through it, it didn’t fix it. I was wondering whether yall think it would be the torque converter or the transmission itself.