I read that you hadn't tightened down the exhaust yet. Sounds like the problem was the MAF, but if you have an exhaust leak before the O2 sensors, outside air will be pulled into the exhaust stream, and past the O2s, which will think that the exhaust is lean. They will then tell the computer to send more fuel, causing a rich running engine, but error codes which denote a lean condition.
'88 GT convertible, 5 speed, 3:08 gears, A9L Mass Air conversion. Mac shorty headers and H-pipe, Magnaflow catback.