There are a lot of factors to consider when swapping an LS engine into a vehicle not originally equipped with one, perhaps the most important being the transmission choice.
The good news is that GM made it really easy for folks to mount an earlier transmission to an LS: the bellhousing patterns are virtually identical and there are no unusual offsets to resolve.
That means you have a ton of options when it comes to mounting a transmission to your LS, including the TH350/400, Muncie, Saginaw, and 700R4/4L60E—heck, even a Powerglide will bolt-up without any significant fabrication.
But, there are a few parts you’ll need to buy and install to ensure your transmission will work properly behind a GM LS engine.
This video shot by the folks over at Summit Racing will go over exactly what it takes to mount an older GM transmission to an LS, including a list of part numbers. Automatic transmissions will need a thicker (or bell-shaped) flexplate and adapter/spacer, while manual transmissions will need a new flywheel with an extended clutch mating surface flange and pilot bushing.
This video is a must-watch if you’re starting an LS swap project, particularly if you haven’t decided on a transmission yet.