1986 Chevy IROC Z28 Camaro

I have an 86 iroc z28 with t-tops. The milleage on the car is 129752 but not the engine. The stock engine was a 5.0 liter MFI and the original owner put a 1974 5.7 liter 350 in with a 1987 tuned port injection manifold. My problem is that the car stalls out at low speeds/at stoplights. Also I am getting around 5-7 MPG right now. It has the stock 700r4 4 speed auto in it. It only stalls out in Reverse, Neutral and Drive, but not in park. I checked the ECM and the ECM # is 1227165 with PROM 8958AMU. I can't tell if the ECM is stock 1986 or a remanufacted ECM. Once the car hits about 500 rpm it starts to loop/chug and then the rpm goes down to around 250 for a minute an then stalls. Unfortunatly I can't put a scantool on it yet because I have not yet located the datalink connector. Original owner redit the entire dash/consol too. I know the 87 Iroc came with a 5.7 liter TPI and I also know the ECMs connectors for both years are the same in fact the ecm is the same ECM 1227165. I have not been able the find the PROM number for the 87 Iroc. I am wondering if I replaced the ECM and the PROM to a stock 1987 IROC Z28 that that would fix my problem. If you have any insight that would be great. Also all the main sensors are plugged in.