Might be 04 then. My 04 has no return and regulator in the tank.

Also, the 305 HP 5.3's newer designed heads and aluminum blocks than the 295 HP ones. As for the earlier ones (275)I thought the cam was slightly different as well as the injectors.

 

I think that either 03 or 04 the t-stat changed from the one that's the whole housing to the separate stat/housing.

 

That's 03 and up.

 

99' 5.3L=270HP
99' 4.8L=255HP
00'-03' 5.3L=285HP
00'-03' 4.8L=270HP
04'-06' 5.3L=295HP
04'-06' 4.8L=285HP
05'-06' 5.3LHO-310HP
99'-06' 6.0L=300HP
01'-06' 6.0LHO=345HP

injector change in 00' to 26lbhr
aluminum heads from cast iron in 01' for all 6.0L's
returnless fuel system starting in 04'
electronic throttle body starting in 03' standard
03' and newer with "quiet pistons"
03' and newer 6.0L's recieved same crankshaft as the H.O. 6.0L standard
02' and up should have no EGR
04' and up have a seperate thermostat from the housing
05' and up 1500 trucks removed the cooling fan from the engine

I'll add more as memory serve's.