Swap Issue

['72 Pro Street]

Hi All,
I am having a problem with my truck after swapping in a 03 5.3L. I have around 700 -800 miles on it now and this issue showed up in the last 100 miles or so. The truck will not start on the first try it just spins and doesn't act like it is firing. I can shut the key off and then try again most of the time it won't start again but will backfire out of the exhaust. On the third time it will usually start. I am getting a code for cam position sensor(P0343) and lean condition(P0171,P0174). I have replaced the cam position sensor and it acts the same. I have done some research and seen were some people were having problems with the cam bolts coming loose. I took the cover off and check them and they are tight. The timing chain seemed a little loose but from what have read that is pretty normal. Just wondering if anyone has any ideals on what it could be? After it starts it seems to run just fine. Sorry for the long just trying to give as information as I can. Thanks John