This page shows what is necessary to convert over to Fbody style injectors on a new body style(NBS) 99+ GM pickup truck with the 4.8L, 5.3L, and 6.0L engines. 

The stock injectors are multec II style injectors which are not as big as the Fbody type.  They also have different connectors and can only be bought up to 43lb/hr @ 58psi.  If you want to push further than those are capable of you will have to step up to the Fbody style injector. 

I bought a new fuel rail and manifold so I could prefab a few things which made the job easier but, it's not a necessity.

This will be typed up assuming you have new fuel rails and a manifold to work with. If you're just reusing the stock rail then of course it will have to be taken off to work with.

The first thing you want to do is take the crossover tube out of the two fuel rail sides, you'll need a Torx T-20 bit to release the hold downs for the crossover tube. When the crossover tube is off put new injectors in the four corner spots in the fuel rails. Then install the fuel rails back in the manifold without the crossover tube. Next comes bending the crossover. This part is kinda subjective but, I have a few pics that will help give an idea of what needs to be done. I just bent the crossover until it looked like the injectors had even gaps from side to side and front to back where they went into the manifold. Basically you want the injectors centered up in the manifold hole. Here are some pics of how you'll have to bend the crossover.

This one is a top down view of the stock crossover.

This one shows how high the stock crossover tube is off the manifold. It's about 7/8" high.

Multec II to Fbody fuel injector conversion

This one is a top down view of the crossover bent for the new injectors.

This one shows how high the bent crossover tube is off the manifold. It's about 1 3/4" high.

To bend the crossover like that I just grabbed it on both sides by the ends that go into the fuel rails and SLOWLY squeezed it together with it bending in the middle at the top. That took care of it width ways but, the passenger side of it still needs to be pulled forward to reach the hole in the passenger side fuel rail since it now sits further forward. To do that I installed just the driverside of the crossover into the fuel rail and held it in the middle at the top of the manifold. While holding it you can SLOWLY pull the passenger side of the crossover around until it's where you need it. The best thing to do is bend it a little and test fit it then bend it some more and repeat until you think the injectors are pretty centered up in there openings. The crossover tube has to be pretty good too where it mounts to the fuel rails or the gaskets that seal it might end up leaking.

Once you get the crossover bent you'll need to take care of the bolt holes in the fuel rails. Since the LS1 style injectors are taller the bolt holes in the fuel rails will have to be reamed out so the bolts can line up with the intake manifold holes again. They're easy to do since the fuel rails are plastic. The only one that presents a problem is the rear one on the drivers side. There is a bracket that the bolt goes through that will also have to be reamed out and it is made of pretty thick metal. The best thing I had to do the job was a rat tail file which took a while to get the material removed. I don't have a pic of that bracket but, will get one to post. On the drivers side you'll have to ream them out towards the front and on the passengers side you'll have to ream them out towards the back. One other thing, get a good idea of where they need to be reamed and take the injectors out and set them aside while you remove the material. You don't want any stuff getting down in the injectors through any openings in the fuel rail.

Here is a pic of what you'll have to do.

Alright once you get all that taken care of you can cut some spacers to put under the fuel rail mounting tabs since they are now about 5/8" above there old location. I cut my spacers out of 1/2" aluminum tubing and made them almost 5/8" long. I filed them down until they fit and allowed the fuel rails to be snugged down just a little bit when the bolts were tightened. The spacers ended up being around .610" tall. You don't need to be exact just leave a little bit(little bit ) of space in between the spacer and mounting tab so the injector is pushed down when the mounting bolt is tightened up.

Here is a pic of one of the spacers.

If you're still thinking about doing this there's more.

Next you have to cut off the old fuel injector connectors and splice in LS1 style connectors. Snip the old injectors off close to the connector so you have some wire to work with out of the loom. You can get the LS1 style connectors from
with pigtails for pretty cheap, $4-$5ea. I got mine from a guy with an old harness laying around so that is why it looks dirty.

Here is a pic of the two side by side. The LS1 connector is on the left and the truck connector is on the right. The wire colors all matched up between the two connectors.
Now all you have to do is start soldering and heatshrinking.   I recommend putting a rag under the wires when you're soldering to keep any stray solder from dripping into the intake if you don't have the fuel rails sitting in yet.  The wires in the picture below look like the colors dont match up but, it is just from the end of the wires being faded.
Once you get the connectors all soldered and sealed up tape any exposed wires with electrical tape and put any wires back in the loom that you may have taken out. Here is a pic of the passenger side retaped and loomed.
When you finally go to reinstall all the bolts and get everything tightened down you'll need longer bolts to put in the fuel rails. They are 6*1mm bolts and you need new ones that are around 1.5" long. I also used a lock washer and flat washer. Here is a pic of the old and new bolts.
Once you get everything bolted back up just turn the ignition on so the fuel pump will prime and check for leaks. If you have any make sure the gaskets aren't pinched and rebend stuff if necessary.

Don't start the truck up until you adjust the injector flow rate in the computer to match whatever injector you put in. Then check with a scanning program to make sure everything is OK.
If you have any questions about this swap you can always ask them at  in the truck forum.