tech ? non mopar

hope this is right thread for this. i bought a 96 s10 with a 4.3 cheap since it needed a auto tranny. well i'm getting a code p0171 , p0174 which states its running lean on the upstream exhaust on both sides. i've checked this out which covers alot , but short i replaced both upstream and down stream o2 sensors when tranny was done and new cat. the mass air flow is new and i've checked for vacumn leaks and can't find one by spraying carb cleaner around intake and everywhere else on top. book states that low fuel pressure could cause this too. i checked fuel pressure at shrader valve and have 40 lbs, haynes manual says i should have 60-65 lbs could it be this simple as pressure. the guy i bought it from had papers for alot of work from a shop and the fuel pump was replaced at one time , but some of the other work wasn't up to standards i would want but thats another thread. the truck runs fine with no hard starts or misses , just this code that keeps popping up. does anyone think that the 20-25 lb differance could cause this code? sorry to ramble but any input would be great thanks tim