Could be an exhaust leak or could be your O2's aren't getting hot enough if they are at the collector end of the long tubes. You didn't say what you were running for a MAF and if it is calibrated for 24#...or if you have a chip or something. If all that is stock, I don't see how you could be running stoich 'cause your pulsewidths will be wrong and you should be running rich, especially since you fuel pressure is 1 lb higher than stock.