1997 7.4L MPI Fuel pressure problems - help me understand

So i've had a surging issue on my 97 330 SS with 7.4L MPI motors. left motor idles fine, but when it gets to 2500 or 3000 RPMs it surges, dying occasionally. i've had our mechanic out troubleshooting, and we were getting low pressure. my initial thoughts were check and replace lines, check and replace filter, since thats the cheap stuff.

He came out, pressure tested, and we were showing like 30-32 so it had low fuel pressure. i'm tryin to understand how this fuel system works based on the merc service manual. Mechanic says there is a low pressure pump, located on the lower left of the motor, and a high pressure pump, located on the back right of the motor. is this correct?

After reading, it looks like there is a pump located in the VST assembly on the low left, so i'm assuming that's low pressure? when he came out the first time, he replaced a pump 818383T, for $270 or so in parts, plus labor. I got a call this morning, saying he ran the boat out and it was still surging, so he thinks its the high press pump, and wants to replace it with #808505 for a cool $700.

A quick google returns 818383T looking like a mechanical pump, not the electronic pump located in the VST assembly like i thought. so, can someone explain how this works, and what i'm missing??

The mechanical Diaphragm pump located on the seawater pump shaft is the low pressure "lift" pump that supplies the tank in the VST. The pump located on the VST is the high pressure pump that supplies the fuel rails the injectors sip from.
For surging issues, be sure there are no vacuum leaks around the intake plenum. Also check the fuel pressure regulator which has a wee vacuum hose on it. Any change to the air mixture due to a bad hose or other air leak can mess you up and act like major medical when it can often be a zero dollar fix by cutting back the hose an inch and reattaching.
Al guesswork on my part as I am not familiar with your vessel. But def look for vac leaks.

And there's this for roughly $80