My 5.3 just hit 300k miles & I need to replace the engine - can I replace the 5.3 engine AND transmission with a 5.7? If so how difficult is this going to be or should I just stick with the 5.3?

 

The "gen 1" 5.7, with the "vortec heads," would be a step backwards in power and technology. The vortec heads like to crack cylinder anyway. The 5.3 is a better power plant capable of more horse power than a gen 1 5.7, and nothing from the 5.3 will transfer over to the 5.7. (intake manifold, exhaust manifolds water pump, etc. IT IS ALL DIFFERENT.) Stick with the 5.3

Unless you're talking about an LS 5.7 out of a Vette or Fbody or you want more power. If that's the case you would be better off looking for a 6.0 LS out of a HD1500 or 2500 series truck. You may still have to modify the computer to accept it. I'm don't know how the newer fuel injection systems are, but I had a 5.0 that I swapped to a 5.7 once upon a time and I had problems with it until I had the the PROM updated. The computer was delivering fuel to the now larger 5.7 based on the 5.0 tune and it was running lean. If that happens you could burn a valve or even a piston. The newer computers may be able to adapt, I would do more research there,...