What year? I have put long tubes on a few GMT400s, used to have them on my 95 Tahoe and currently my brother's 97 K1500 truck has them as well. Both had the 350 rather than the 454 though.
I would imagine it wouldn't be much different than finding headers that will fit the Gen IV/V/VI big blocks and then running your own custom exhaust after. Mostly what I had to do on the 350s. I did have to make my own O2 sensor port by drilling and then welding in a bung at a place that would work better for the sensors on the GMT400. The harness is not very long on the 95 and earlier models so the stock one in the headers would not work. I just sealed the original one with some welding as well.
For the 96+ Vortec, they have 2 O2 sensors on each side for the front sensors as it is OBD2. Had to make the harness longer on the passenger side for it to work and add a O2 port on that side as the headers only had the one port for the driver side. You will also need to disable the rear O2 sensors via reprogramming the PCM or tricking them with an O2 simulator, otherwise you will get a SES light unless you plan on running a cat converter and putting O2 ports in your exhaust pipes after the cat converter.
I also had to create my own EGR pipe on the 96+ Vortec so as to not loose the EGR function on the truck. I did that by using the original driver side O2 port that comes on the headers since I had to weld in a new one for the actual O2 sensor there, ran some iron piping up and connected to the EGR at the back of the engine. Sealed it up with some high temp exhaust rated sealant to prevent leaks. Not a fun job to say the least but at the time I did this on my brother's truck I didn't have the ability to reprogram 96-97 GM truck PCMs and you do not want to run the truck without the EGR if you don't go in and reprogram it for this. It will throw codes and it will run poorly as the timing is adjusted when the EGR is in use.
You can avoid all this hassle by having it reprogrammed to disable the EGR codes and remove all the EGR timing adjustment, if you can find someone with the ability to reprogram your truck. 96-97 is hard as it uses a PCM that was only for those years, 98+ are easier as more programmers can do those. About the only one I know that will do the 96-97 years is the original Tunercat OBD2 system, which you can't get anymore and was expensive even when you could. There may be others these days but I don't know if there are any good ones for a reasonable price.
Other than those things, its just a case of making your own exhaust after the headers are installed. There are various kits but none will work perfectly, you will end up having to fab up your own pipes and hangers to make it work. I ended up just dumping the exhaust before the axle on my Tahoe as it was a pain to get a dual exhaust over the axle, there just wasn't room for it. My brother's truck had no issue though, more clearance back there with the long bed he has. If you have the skills and tools, it can be done. But if you aren't willing to spend the time and effort on the custom stuff, I wouldn't bother.
