A couple of things.
The Main Beam (front crossbar) functions as a truss with the vast majority of the vertical mast load being supported by the tension member, the Dolphin Stryker V-Bar.
The lateral forces are controlled by the holes through the Main Beam.
The Dolphin Striker Casting is held in place by the pressure(pre-bend) from the Dolphin Striker Rod.
The Compression Tube prevents this pressure from collapsing the Main Beam.
Unfortunately, pre-bend reduces the height of the Mast Ball above the Main Beam.
This can cause problems with the clearance between the Mast Base Casting and the Main Beam during mast stepping.
I have seen some Mast Base Castings gouged by the top Dolphin Stryker Rod nut.
Adding a plate to the top of the Main Beam will further reduce the limited clearance.
Hopefully, you will have enough clearance on your boat

I would use a temporary bolt to hold the new Compression Tube in place and to pull the new plate in tight to the Main Beam while it is being welded.

P.S.
I think the 5.5 is 8.5' wide, but the 5.2 is only 8.0' wide so the crossbars won't work.