It would help a little to know which trans you have. The L6 Glide has different ratios and is generally believed to be weaker. If you have the base 3-speed manual, it also has different ratios, but I don't think strength is as much of an issue.
Just understand that is is a "snowball" project; you can make some things work with rigged up kind of fixes, but it will take more money and time to do it right. If you really want to do it, you'd better do a lot of research or have deep pockets to pay someone to do it; if you "don't have a clue" and you want to jump right in without thinking things through, you're just creating the next "I started an engine swap, but I couldn't finish, so I am selling it cheap..."
If the car is an original L6 car, it likely has monoleaf rear springs and a non-posi, low numerical ratio rear axle. None of these are desirable behind a strong V8, but might live for a while depending on how it is driven.
A small block V8 car uses different engine frame stands (bolts to subframe) and engine mounts (bolts to block) than an L6 car; these are available used OE or repro. You may want to read up on the different types of mounts used in the different cars http://pozziracing.com/camaro_engine.htm
. If you intend to run a later model 350 with a large (8-inch) balancer, you need to use a certain combo.
You will need a shroud and likely a new radiator. You can probably try rigging the engine and front lighting wire harnesses to work (my L6 69 vert had been rigged to run a small block, but the wiring looked awful), but it is better to replace them with the correct small V8 style.
There are many threads here on this forum regarding this exact question, because it is a commonly asked question. Try typing "small swap" and see if a bunch of relevant threads don't come up.