904 transmission blows 2nd to 3rd shift under heavy power

My son said this happened to him the other day. I thought maybe the fluid level was low. He said it seemed like it slipped when shifting from 2nd to 3rd when he was hitting it hard.

It's a 1974 Dart Swinger with a newly rebuild 360 and a 904 bone stock transmission. Small rear end 7.25" I believe.

Anyway it did the same thig to me today. I checked the fluid levels, it's at full warmed up in neutral. I hit it to get on the highway, everything was going well till I hit about 70mph with the 4 barrels still wide open, then it bucked like it lost all power and then jumped back in it.

Thoughts? I've neve been inside the transmission. It was with the car when I bought it. Originally a slant 6 car, so I assume the whole drive train is from the slant 6 era.