When my kids were in college, I gave each a 2nd gen Firebird to commute with. One was a 1979 Formula with 301 and T-350 trans. Since it had 150k on it, I had the 301 and T-350 re-built. The 301 was re-built by a real engine shop that does professional work. The T-350 I used a local tranny guy and he said there is a $200 rebuild, and an $800 rebuild- which do you want? I went for the $800 rebuild because I already had a $200 rebuild fail after two years. That was over 20 years ago. That T-350 is so solid that it is still being used in a 1980 Trans Am w/400 right now. It still works perfect. The 301 is still powering my 1983 G-body wagon. You get what you pay for. Since they haven't made a new T-400 in decades, it would be unusual to find another original like yours. Most will have been re-built, often several times. Once diagnosed, if your T-400 needs a re-build, I would just park it, save up, find a competent shop, and consider it an investment. You will never get stranded or have to worry about it again. And if you ever swap over to a manual trans, you can re-sell the T-400 easily.