It depends on what year truck you're putting it in. The motor mount plates i think are the same for all 5.7 to LS style swaps in our year range. You can use the stock exhaust manifolds if available with the engine, just make sure you also got the 8" pieces that have the o2 bungs. What are you doing about transmissions? If you reuse yours, whatever kind it may be, i think you'll need some sort of adapter/spacer for the crank shaft or fly wheel. If your truck currently has a 700r4/4l60e, and the new motor comes with a tranny, use the newer tranny. I had a 700r4 with my 5.7 in my 1990, and my drive shaft and trans cross member worked great with the new 4L65E that i put in with the new motor. A lot of people say to use the 1" set back motor mount adapter plates. I did, and ended up having to drill new holes for the trans cross member 1" behind my original holes. No big deal. Since the swap, I've swapped out my stock exhaust manifolds for the schoenfeld long tube headers for LS swaps into 88-98 pickups (1302LS1 i think is the part number). But now my headers are rubbing the fire wall heat shield. So, when i get a good long weekend, I'm going to swap out for the original adapter plates (which is what schoenfelds website says are compatible with the headers), since I've got some beefed up 243's that have been waiting to be installed, and I'll already have most everything out of the way to do it. You said it's a99 silverado, so it should have the mechanical fan on the front. That's fine and will work good with your existing fan shroud. But there's a ton of other factors you need to think about. I believe 97 and 98 trucks (i could be wrong) have fuel pumps that's flow enough to feed an LS motor. Otherwise, you'll need to swap out for something like the acdelco ep381 (like i did), or something else like a walbro 255 or aeromotive stealth 340 (aeromotive #11169 is what I'm switching to with my head swap and supercharger install. It's a direct fit replacement). You'll need fuel lines and adapters to go from your fuel filter & return lines up to the new fuel rails. I could go on and on about this for hours. A lot depends on what year truck you're swapping into. Later years (95-98 i believe, but could be wrong), require you to keep your existing pcm and have the harnesses mated together. Our i believe you can have the 411 pcm flashed and set up to run an LS, and your stock gauges and whatever else. If you want to keep ac, you need to decide whether you're going to run the 5.3's ac compressor, which will require you to notch and box the frame to fit. Or you can get an adapter bracket that retrofits your trucks existing compressor to the top passenger side. That's the route i went in my truck, since i don't have access to welding equipment. And you don't necessarily have to "Crack" the system,if you have a good working ac system. And you'll also need the appropriate spacers for the bracket. Not knowing what year your truck is, I'm just going to tell you what i did for my 90. I got a pcm tray, drivers side battery tray, under battery washer bottle, and throttle cable out of a 97 suburban. This allowed be to run the harness pretty close to the way it is from the factory in the newer trucks. And allowed me to hook my pedal to the newer style throttle body. I left my passenger side battery tray for my k&n filter to rest on. Anything else I'm forgetting, I'm sure someone will chime in. Miscellaneous sensor adapters (coolant temp, oil pressure). You can use the radiator hoses from the 5.3 on your truck. Try googling gen III 5.3 swaps into your year truck, and you should find plenty of info. These swaps have been done a million times over, and have been heavily documented all over this website, and other websites like ls1tech.com, and ls1truck.com. good luck, and i hope some of this helps you. Here's a video of my truck shortly after finishing the swap.
https://youtu.be/0raqaxJTy5U
I am jealous!
