I put in a marine intake early last year. It is a superior intake to the stock vortec, there really can be no question on that. It is free of all interior obstructions and absent the spider has an enormous volume which is advantageous for torque building. It has a standard fuel rail system which means you have many options regarding fuel injector selection. Having said that, for a truck motor the end result performance gain is pretty minimal. It's not nothing, but it's minimal. I would expect it to be a more reliable fueling solution (look at all the posts of people who have issues with their spiders vs the hundreds of millions of vehicles with standard fuel rail injectors which run without issue through their service lives) which was my aim when I went through my motor last year. Lastly, if the spider had any advantage over fuel rail injectors why didn't any other manufacturer, including GM, copy it? If you do get an intake try to get it with the accessory bracket (kind of an S shaped steel bracket that spans the manifold) because it makes it much easier to mount the canister purge valve and attached wire loom to keep the install clean. If you convert to cpc ignition at the same time as I did you won't have to worry about the ICM mounting either and what you'll end up with is a cleaner than factory installation.
With regards to your questions:
-Do any of you know the differences in those specific 4 hole throttle body 5.7 vortec marine intakes? No, mine had the same 3 hole tb as stock but on the PPED forum there is some discussion about getting an adapter for it. Given the dates of postings though there may not be adapters available any more.
-Am I wasting my time trying to install the marine intake to begin with as its not going to see high revs most it's life? You don't need it since the stock manifold is sufficient. But as mentioned it is superior to the stock manifold. May be easier to tune too.
-If it's worth swapping over, can you interchange either the top half of the intake? The top halves are not interchangeable. Beyond the big hole where the spider penetrates the upper manifold the stock also has carveouts in the upper manifold for pcv and canister purge valves. So they're not the same shape.
-or do they make a 4 bolt to 3 bolt throttle body adapter that I can't seem to find? As mentioned, take a look at the PPED forum. I know it was discussed.
The EGR tube is really easy to fix. You need two 5/8-3/4 flare adapters, two 3/4 flare nuts, and one 5/8 flare nut. The stock EGR tube is 5/8 and the middle foot or so of it is smooth SST so you cut it in the middle and flare it for a union. The 1999 Silverado uses a "divorced" EGR which can be mounted outboard of the AC compressor (basically where the pulley of a whipple sc would go if you had a whipple sc). Cut the adapters off of the 1999 EGR tube ends which leaves enough SST to flare (the 1999 EGR uses a 3/4 tube). Feminize the 5/8 end of one of the 5/8-3/4 adapters and put it in the manifold and then use the other adapter and flare nuts to put it all together. If you do the EGR this way you can keep the stock coolant hoses. Never could figure out why the author of the original procedure thought you needed a radiator hose from a pre-96 truck anyway.
The marine intake isn't hard to do and you don't have to abandon anything a stock truck has to put it in. You can buy new 24# Denso injectors for about $150 for 8 on ebay which are intended for use in marine engine overhauls (they made a lot of Marine Magnums). My intake came off of a 2005 Mercruiser with a 305 and cost $550 to the door. Probably spent an extra $400 on everything else (new injectors, injector harnesses, wire, 16 pin metripac, vac tubing, thermostat, canister purge valve, water neck, tb spacer, egr fun (not fun - easy solution after a half dozen failed solutions), etc). So it isn't a cheap thing to do. But it does work exceptionally well done right. Is it worth it? I have no regrets but it certainly isn't something you need for a 383. But by that logic a 383 isn't something you need either.