Welcome to the board Eric.
1990 was the last of the 1977-1990 body style before they went to the inverted bath tub for better mileage. A 305 or 350 motor size will be labeled on the radiator core support as either 5.0L or 5.7L. They shipped both size motors. The 305 is the HO with "E" heads that have bigger ports and smaller chambers than the usual 305 motor. Tranny should be the 200R4, and the rear end will be the 8.5 inch rear with an open differential and 2.73:1 rear gears being common with a 3.05:1 being the steepest they offered without a towing package. The 8.5 will bolt into an A-body but it is three inches wider than the Chevelle rear end so you will need some creative wheel offsets to get it to fit. Also the B has the springs mounted to the axle and the A-body mounts them on the lower control arms. So all of the bracketry will have to change.
If it is a 9C1 car it will have an electric trunk release, oil cooler, tranny fluid cooler, and a P/S fluid cooler installed. Remote left and right hand mirrors and a knob on the dash like a manual choke to set the idle to a fast speed to keep the 185 Amp alternator spinning at a higher speed to power the "auxiliary" lights. It should also have a radio delete plate where the "juke" radio went in a passenger car. The chassis will have HD radiator, HD Shocks, 12"x1" vented disc front brakes, and 11"x 2" rear drum brakes with special HD steel wheels to clear the larger disc brakes mounting on a 5" bolt circle with 1/2" wheel studs instead of the normal 7/16th inch studs mounted on a 4-3/4 inch bolt circle.