How to test fuel pressure 454 mpi L29 engine

[Salmonhound]

Hoping for some help. I have a 454 mercruiser mpi L29 engine, serial # OL002958. I am having problems with a rich running motor and some loss of power. Pulled plugs and two had a fuel smell to them and the bottom 1/8 of the plug appeared clean, without carbon, my guess it was sitting in fuel- my opinion.
I would like to start with checking the fuel pressure while running and ignition on but I can't seem to figure out how to attach a fuel pressure gauge to these motors. I downloaded the 1k page manual but they don't explain how/where to attach the fuel pressure gauge either. Could use some friendly advice and help. Let me know if you have further questions.

 

[alldodge]

Should be right close to the TB, its shows item 6 is where the schrader valve and cap cover

https://www.mercruiserparts.com/bam/subassembly/31671/2713/130