Sounds like a cool swap. Modern powertrains in old trucks is just way cool.

I have seen a 1972 K5 Blazer on a 1988-1998 short bed 4wd chassis before and it didn’t look quite right.

One issue you would have with the 2002 chassis under the 1978 body is the wheel base is longer on the newer trucks by close to 6”. That could easily be dealt with by cutting off the rivets on the rear axle spring hangers and sliding the axle forward to line up with the wheel well opening on the ’78 body and reworking the rear driveshaft. But the next issue is with the track width as the newer trucks have a much wider track width. It would look silly with the wheels sticking that far past the fenders and even worse if you run aftermarket wheels with a heavy offset. Plus you are into off-roading you would have a chassis with the less than sterling IFS front suspension.

Mounting and leveling the body on the 2002 chassis would be a challenge of its own as well the complexity of selling the truck with 2 titles if you ever decide to as many states get weird about matching body and chassis VIN’s. Potential buyers often get spooked about these type creations as well.

I would recommend just installing the powertrain into the ’78 and call it a day. As you probably already noticed, you won’t be able to use the 261 case with the ’78 front axle though as the transfercase front output shafts are on opposite sides. A 241 transfercase out of an late 80’s or early 90’s Blazer or Suburban should bolt right up to the 4L80E or you can get an adapter from Advance Adapter$ to bolt a NP205 to the back of a 4L80E. Or another option would be to convert the ‘78 over to a Ford Dana 44 or 60 with the left side pinion and keep the 261.

I recently swapping an 8.1L in a ’78 K10 as well but kept the original SM456 manual transmission and 205 transfercase. The 8.1L was a direct bolt in whereas the LS engines will be a tad different but easily achieved with engine mount adapters.

L18 8.1L - a set on Flickr

As SVIC said, the 67-72chevytrucks.com is a good resource as there a lot of people doing LS swaps into these old trucks.