|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.