I have a 5.3l '01 GMC Sierra with a head casting number of 862. I can get a set of LS6 heads with casting number 243 for about $200. These came from a CTS-V that has been upgraded to about 550 hp. Would this swap be worth the time and effort. I'm looking for more power and preferably torque. I can also get the cam from the CTS-V if that would help out hp and torque at all. In comparison, my '03 Suburban has the 5.3l with 706 heads and feels like it has more power, especially when towing. Any help would be appreciated.

 

For a direct swap without any porting it's not worth it one bit. The 243's have a 64cc combustion chamber and the 862's have either a 61cc or 62cc can't really remember but anyway you won't notice any difference in power. Now if you get the 243's ported you'll get a decent gain of power, the stock CTS-V cam is nothing to brag about sitting @218/210, you could easily go with something bigger and more streetable.

 

I say buy the heads and flip them. You can sell them for about $500

 

If you're looking for more torque for towing, you've got 3 options:

1. More displacement.
2. More compression.
3. Forced induction.

As stated before, those heads will improve flow, but reduce compression. If you had them milled down to 58-59cc (no need to port) you would see a noticable improvement, but you would most likely need a custom tune and it would require 91+ octane.

You failed to metion what year CTS-V. The early cars had the LS6, and the later had the LS2. The cams are different, but neither will really improve low end torque.

If you're willing to give up torque below 2500rpm, there's alot to be gained in the midrange with those heads and cam

 

This might be a good swap if you wanted to maintain power and switch to lower octane fuel...
But you won't see a huge power improvement ( if any ) from switching to untouched 243's on your truck.
Get them worked on though and that's another story altogether...

I paid about 500 bucks for my 243's, flip um.