LT1/4 hard starting

[dave69]

I swapped a 96 Vette LT4 back in Nov in my 69 C10 and it has been running great every day doesn't skip a beat, except lately it has been a little hard to start. It will start and run most times but when it won't it will pop at the engine (sounds like a back fire),but not out the exhaust and may be enough to blow off a vacuum cap and than crank for bit then start and run great. The next start fires right the odd time it will pop and crank awhile than start. I checked for codes and it has 2 P0135 and P1133. Do I need to change one of the O2 sensor? Not sure what is wrong.
Does anyone have some advice on what I should be checking and or replacing?

Thanks Dave