Your fuel pressure should be 30 psi
Pressure is read use adapter 91-806901 placed inline at the TB or by connecting gauge in place of item 1
https://www.mercruiserparts.com/bam/subassembly/31886/4856/130
Easiest to do it with boat off, can be done with it on, just harder
Yes, keep leak springs and should be no problem so long as your axle is similar as old one
Many are the same but many are not, there is the rub
Unless the auto and marine are tested side by side your guessing. There is a guy on Offshore only "articfriends" that has a shop in Michigan, could ask him
Why do you think you need new ones?
Are you talking about the "retainer" item 28 part number 3858970?
https://www.volvopentastore.com/Fuel-System/dm/cart_id.456747459--session_id.074675529--store_id.366--view_id.775644
That would be a good idea, provide them engine and PCM numbers
In post 2 you listed your serial numbers but when I look up 1A437598 it comes back as invalid at mercruiser parts, and comes back as a Bravo drive at Mercury site
My guess without knowing more its your Oil pressure switch
Crank the motor and power goes to fuel pump and fills carb. Motor is running and key is released so now "IF" the oil pressure sensor re-powers the FP then it continues to run. The oil pressure switch is not making contact so it runs...
You did post the PCM numbers prior, I just missed it
Both part numbers show up on Mercury Parts, but neither show any motor listed for them, 8M2015889 has no superseded part number, 8M0031299 shows a prior number of 8M0071570 and list PCM 14.
Has the PCM part number been verified it's the correct one?
Reading it shouldn't work with Axius removed, wonder if they were changed out
I'm finding 879194T17 for 377 Mag, closed cooling Axius