If you were showing O2 codes it would more than likely be your cats. If your cats were bad and you sealed up the leak the excess back pressure could potentially damage your engine and it would start leaking again. But you do not have this problem. So weld up the hole.
new engine,1.7 rocker arms,Pipes exhaust, shorty cats , shorty headers,under drive and overdrive pulleys,4.2 upper plenum, throttle body spacer, cold air intake, stage one clutch, shorty shifter, and tune. 8.8 in storage