TH-350 slipping? TH-400 swap?

Do you have the kick down cable hooked up? That increases the line pressure. Forget the TH400. I have a TH350 in my 8sec 3700lb Chevelle. Everything stock except for the valve body and int sprag.

Also check the fluid level. Too much is worse then too little.

Yes you are correct. Don't just pull the cable out and drive though; it will never shift.

And that could be part of the reason why you are having problems.

Kick-down cable

Thanks, as a matter of fact I have been fiddling with the kick-down cable because it was holding the throttle back from opening all the way. I am using one of those one-size-fits-all cables and am having trouble figuring exactly where to lock it down on the cable. How can I test the transmission to know for sure it is the adjustment of the cable and not something internal?

I always heard the TH-350 was stout enough, but if this one is shot then it'll be my second. Excuse what is obvious to you, but is that 8 sec. 1/4 mile or 1/8th?

By the way this is a TH-350C (w/ lock-up), but I haven't hooked that up, yet.

What you want to have is the cable pulled all the way out at full throttle.

Yes that was 1/4 mile time.

I don't know much about the lockup type 350. I know on the 700r4 and 2004r, if you don't use the lock up on the highway, you can burn up the trans. How does your fluid look and smell. Is it burnt?

You need to find a good tranny builder and put in a non lock up one in.