i have a 1997 chevy k1500 5.7 and i keep getting these codes one is co502...po327....p1345 i just did a tune up on it and changed the oil what could be making these codes pop up and which one do i have to worry about and also when i drive it wen i give it gas it backfires and wen i let off it quits help me please

 

P0502 is VSS(low input)
p0327 is knock sensor low voltage
P1345 is cam\crank position sensor correlation

I would start with P1345 before anything. Check your timing to make sure it is good, make sure the distributor is installed correctly.
If all checks out ok your probably going to need a crank sensor.

The other 2 codes are probably caused by the engine not running properly so once you get it running right, clear the codes, and check to see if anything come back