I just got my car done 350 is in, only thing it rumbles when I accelerate.
It idles at 500-750 rpm and the SES light came on after driving for half an hour.
How do I check the code with a pin, I forget.
Im using the old O/2 sensor from the 305, would that matter?
Any ideas what the cause is, maby timing?

 

Found out about the codes, I have a code 44 "lean exhaust".
How do I fix this?

 

Likely that old o2 sensor. Those things don't like to be handled much. 305 or 350 o2 makes no difference. Same part. You might have a vac leak causing it to run lean.

Wouldn't hurt to check your timing but I don't think thats it. I put my money on the o2.

 

Could that cause the weird idle sometimes and rumble under the car when I accelerate?

 

Yes it could cause all kinds of strange things. It's the most important sensor on the whole engine. If it's reporting back bad data, it will make the engine do all sorts of weird things.