I am fairly new to the board and decently new to working on cars. I used the search function and couldn't find a clear answer. I have a 84 z28 with a th350 and 9 bolt rear. I want to use an ls1 driveshaft. Will the driveshaft need to be lengthened or shortend? Will I need any special bolts or a custom yolk? Any info would be helpful. Thank you!!!

You will need Autozone part #3-3130 or equivalent for the u-joint. Yoke is fine if that u-joint is used. LS-1 alum driveshaft is the same length as the steel 3rd gen shaft, but whether the length needs modified for the th350 you can either do a search or someone else can chime in.

you won't need to do any changing of the u-joint for the 9bolt rearend. if you were using a ford 9", then yes, the u-joint needs to be changed. if the th350 you're using has the longer 9" tailshaft, then it is a direct bolt in replacement, if you're using the shorter 6" tailshaft th350, then you'll need a 3" longer shaft.

i have an ls1 driveshaft on my firebird with a 3.45 9 bolt rearend under it.

You're right, but if the existing u-joint that comes with the driveshaft is in bad shape or you decide to replace it anyways, the part number above is what you will need.