Check engine light

Did the problem start after the repairs? If so check your spark plugs and see what they're telling you. If there's a code you can retrieve it with a paper clip and counting the flashes of thr check engine light, im sure there's a good guide online on hoe to do so on your vintage of Chevy. Chances are your timing is off by a few degrees but if you're confident its good and that the spark plug wires are good then you have more testing to do. How long after being ahut down is it cool enough to run well again?

If the check engine light is on, pull the codes.

The problem started before I put thr new parts in thats y i put the new parts in to try and fix it but its still throwing a code 13 which is o2 censor but its brand new but i also cut the muffler off to straight pipe it could that be the reason

There may be another O2 sensor that is on the manifold. Normally, Chevrolet has at least 2 or three. Also these systems rely on a certain amount of back pressure to maintain fuel & air ratios.

It only has 1 02 sensor on the y pipe and so I should put some type of a muffler on it

It only had 1 o2 on the y pipe when I bought it and it ran amazing until the fuel pump went

If you did the exhaust modification at the same time you did the fuel pump, and it acted up at exactly that time, put the exhaust back the way it was before it messed up.

There should be 2 sensors. One before the catalytic converter (upstream), and one after it (downstream).

There is no cat and there is only one on y pipe

There's a problem. When the computer can't compare values between the two sensors, it has no idea how to adjust the fuel/air mixture. Unless you plan to put a carburetor on it, you are stuck with the computer-controlled system.

But what I domt understand it rand fine for like 2 months before the fuel pump went then it all went to shit

Did you remove the muffler at the same time you did the fuel pump? You might recheck the wire on your new sensor to make sure the pins aren't bent on either part of the two ends.

I agree with Ninja chicken but seeing as how the ignition control module is in the distributor you can probably rule that out but where is the ignition coil? Is it the Chevy kind thats mounted on top of the engine? Could be breaking down when hot. Jeff his truck is OBD1 and only has the one O2 sensor (front) that adjusts the values, rear O2 sensors monitor the efficiency of the catalytic converter and rarely are programmed for finite adjustments. Also backpressure in an exhaust is NEVER a good thing (even factory exhausts flow fairly well) and is often confused with the velocity of exhaust & exhaust scavenging which go hand in hand.

So i just looked up gm obd1 code 13 and what it is is a failire in the oxygen sensor circut itself. You probably have a broken wire or bad connector. Some older computer controlled vehicles even have fuses for them but thats generally for a heated O2 which yours is not. I hate to say it but you spent alot of money on parts before doing your research, google is your friend theres tons of info out there. Last but not least your main harness connection it could be corroded or ecu may be getting punky, a visual check of the board itself may yield an answer in the form of a swollen/burnt capacitor or alot of corrosion. Good luck to you electrical in older rigs can be a nightmare, im going througj my own atm with my 94 b4000