A pressure leak at the ALDA, within reason, isn't of the slightest concern. The boost pressure is more than sufficient to overcome the leak at the rate of 1 psi/sec and the ALDA won't notice the difference.

What you will notice is the much better driveability (and possibly some increased performance) if you simply remove the ALDA and plug the boost line.

It's a little tricky to access on the 603 but two wrenches usually does the trick. You place one wrench on the bottom flats of the ALDA and the second wrench on the nut beneath the ALDA. The upper wrench should be positioned so that it is slightly clockwise (looking down) from the lower wrench. Then, squeeze the two wrenches together with one hand and the ALDA will break loose from the nut.