Since you've decided to do an LS swap, now you have to decide if you're keeping the original transmission from your vehicle or bringing the modern LS transmission with the engine...or even another avenue like a ZF 8HP unit.

The key factor is usually if it'll fit in your tunnel and if your crossmember can be moved to pick up the transmission mount. Next thing to consider is if you have a mechanical linkage to the transmission. If so then retaining its factory position is typically the better choice. Another benefit of doing so, means you will not need a new driveshaft, although the passenger side cylinder head on a LS engine will sit roughly 1" closer to the firewall than a SBC engine would due to the bellhousing flange to cylinder head distance being different.

We've assembled this transmission size guide to help you compare several transmission dimensions to determine fitment in your vehicle. An example comparison is a TH400 versus an 8HP90 (with bellhousing adapter):

Mount location - You'll notice the trans mount is just over an inch further back (27.86" vs 26.78") and almost an inch lower (3.01" vs 2.15"), so you'll need move and/or modify your crossmember to accommodate that.
Trans Body - The body of the 8HP90 is more than an inch taller throughout the body of the transmission (6.11" vs 4.45" at the front and 4.16" vs 3.0" at the rear) and just over 3" wider (12.0 vs 8.9") at the base of the bellhousing and 1.5" wider (8.1" vs 6.64") at the rear.
Pan Width - 8HP90 pan rails are narrower than a TH400 with 6.0" vs 6.74" on the passenger side and 5.7" vs 5.92" on the driver side, which may improve header clearance.
Pan Depth - The factory 8HP pan will hang 1.6" lower than a factory TH400 pan (8.65" vs 7.05").


What This Means To You
If you know what OEM transmission(s) fit in your factory tunnel or how much additional room you have from any one of these, you can use this comparative data to determine which other transmissions will also fit or require modifications.

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