What is the best engine to replace my 91 350 TBI engine in my 91 gmc single cab stepside. I want more power but dont want to break the bank

Idle air solenoid and/or intake vacuum leak.

check fuse relay wiring and switch and if lighter kept blowing fix dont throw in bigger as you see could have burnt truck down

CODE PO155 - HO2S 21 BANK 2 SENSOR 1 HEATER CIRCUIT MALFUNCTION CONDITIONS.CHECK O2 SENSOR FUSE AND WIRING CRCUIT BEFORE BUYING A NEW HO2S.IF SENSOR BAD REPLACE IT.THEN YOU CAN CLEAR THE CODE.

You can use any 305 motor. Like you said you will have to exchange parts from your old motor to the new one. Just make sure you don't go to far back, because the old 305's don't have the injector ports. Then you would have to replace the heads also.

Yes any TBI motor will work, You do have to switch your sensors, possibly motor mounts and balancer from the old motor. all other items should transfer.

Untill recently GM didn't change much of anything as far as bolt paterns You could bolt up a 350 from 20 years ago to your transmission and motor mounts. But that is not really what your wanting to do. The question is do you have a "vortec" or is it TBI. The vortec motor says vortec all over it (they were pretty proud of it I guess) The earlier TBI motor, at first glace look just like the old carburetor versions until you take of the air cleaner and find a throttle body instead of a carburetor. The two are not interchangeable as to the electronics, they are way diffract.

Oxygen sensor[s],map sensor and fuel pressure regulator.Although you should have a check engine light "on" with any sensor malfunction.I would look at fuel lines for a leak under vehicle.

its the fuel pump or the fuel pressure regulator,both and easy replacement if you know how, pump is about 78.00 at auto zone regulator is about 45.00

P1133 - Heated Oxygen Sensor (HO2S) Insufficient Switching Bank 1 Sensor 1
I would have the repair shop see if they either damaged or left disconnected the oxygen sensor for engine cylinder bank one upstream sensor, this sensor would have been disconnected if the oil pan was removed.

yes