This is a repost from LS1tech.com:

Just a heads up for anyone who might be putting a Gen III engine in an OBS (1988-1998) GM pickup, you can use the motor mount adapter plate plans that are floating around the internet (sorry, this is not very descriptive) along with your stock mounts and some light modifications. The adapter plates have the 4-bolt pattern to mount to the new engine and the 3 bolt pattern to mount to your SBC motor mounts superimposed on each other. This is what my adapter plates look like before painting:



My SBC motor mounts had a raised area in the center of them that had to be removed to fit flat onto the mount adapter plates. I used an angle grinder with a cutoff wheel to hack this raised portion off. I then bolted the adapter plates to the new engine and the SBC mounts to the adapter plates using nuts on the back of the adapters to secure the SBC mounts. This required some very minor clearancing on the webbing that reinforces the motor mount point on the engine block, so threading the holes in the adapter plates would have been a better idea. Anyway, here is a picture of the mounts installed:



That is looking at the passenger side mount from the back of the engine. The mounts place the back of the engine further forward than with the old SBC. This turns out to be a good thing for me for two reasons- it gives me more room to tighten the engine to trans bolts, and it allows me to use my stock driveshafts since I am also putting in a longer 4L80e transmission with this engine. The center of gravity will be moved forward a little bit, but hey, it's a pickup and the weight distribution is horrible in the first place. Here's what the engine looks like sitting in place:



Everything seems to be fitting just fine so far. The exhaust on the drivers side might be a little tricky, one of the studs is very close to the frame and might have to be cut down a little to fit a nut on there. We'll see...