What would cause this?

[Americanmuscle13]

A small update on my transmission issues. Here's the background: Used th350 had a couple shavings in the pan when I got it. It was what I could afford at the time, I put a stage 3 B&M shift kit in and 2000 stall on it. (This kit eliminates the vacuum modulator) I had a mild 350 in front of it, and it work with no issues. Fast forward I've got a ~450 horse 388 I just finished building and dropped in. I put a used 3500 torque converter on, not sure what brand. Now the car will hardly move most of the time. When I putt around the driveway, occasionally it will launch hard with just a little bit of gas. Most of the time it takes 5500 rpms to just move the car in low gear. It's full of fluid, I've tried messing with both the shifter linkage and the detent cable. Neither seemed to make a difference. Could the new motor be enough to cause a worn tranny to slip like this whereas the old motor did not? The occasional hard grab is what confuses me. I realize it could probably use a new transmission anyways, especially with my horsepower output. I would just like to know where to start when I get home in August.

[Twisted_Metal]

Does the fluid smell burnt?

The motor/tranny combo isn't the prime suspect.....
I'm thinking it's a ballooned (ruined, toasted, junk) torque converter.
Neither the shifter linkage or detent would cause what you're describing.

If it isn't the TC... you've got severely toasted clutches in the tranny.
A quick sniff of the fluid should indicate the problem.
(Dropping the tranny pan and finding toasty flakes of clutch material will indicate a tranny/clutch problem.
No flakes... I would be suspicious of the used converter. )