New member, please help. 89 gt new exhaust running rich.

i have an 89 gt with a few slight mods. it has a cam, cold air intake, msd coil, 3 row radiator, and an electric fan. this past week I had my new exhaust put on. it consists of mac 1 5/8" long tubes, 3" catless h pipe, and 3" cat back. Before the exhaust was put on, the car ran perfect. after the exhaust, the car ran rich. I was able to smell fumes when I was idling and sometimes when I drive. one of my buddies recommended swapping the 02 sensors for heated 02 sensors. he said that the 02 sensors required a specific amount of heat from the engine that got reduced when the long tubes got put in. need some help. any recommendations?