The 4L80 will not bolt up to a LT correctly, you'll only 5 bolts engauged have which works but not ideal IMO. GMPP makes an LT to 4L60 bellhouse to get the top bolt to mount Chevrolet Performance - 19125817 - GM LT Gen5 4L Auto Bellhousing . I wouldn't run a 4L80 anyway as it will eat to much power up IMO. A 4L60/70/75 will be fine behind a 355hp L83 I would still upgrade some internals as mentioned.
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