You can buy a boost leak tester for a few bucks but will need an air supply, works on NAs too to help find vacuum leaks. You should go over your 6 P's before you go any further down the rabbit hole and waste more money on parts: [ http://twinturbo.net/nissan/300zx/forums/technical/view/922678/The-6-Ps-Proper-Preparation-Prevents-Piss-Poor-Performancegt.html ] There are similar problem diags found in the service manual: https://www.300zx-twinturbo.com/cgi-bin/manual.cgi?list=ef&slide=42 That rust spot is probably from a loose or clogged evaporator drain, see it pretty common on our ECUs.
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