Engine/transmission query Luhrs 290 Sport

Appreciate any help sorting thru a possible engine or transmission issue. Both engines rev fine, well up to 7K+ RPMs when not under load; however, under load my starboard engine revs ~2500 RPMs half way up on the throttle control and then there is no further output/propulsion the rest of the way up throttle. Port engine will push 4,650 RPMs but the starboard engine only generates 2500 RPMs. I'm perplexed, here's why...

I have twin Marine Power 5.7 carbureted 350hp engines. I've done a complete overhaul of the motors and major components other than replacing the block. Have not done any forensics on the hurth transmissions. Both engines have new harmonic balancers, new manifolds, risers and exhaust plumbing, new seawater pumps, new distributor cap and rotor, new engine wires and spark plugs. All heads on both engines have been pulled and refurbished (port engine had a cracked head and was replaced). Proper adjustments made to all valves, timing is set to 10 degrees at idle and advances to 29 degrees at 2500 rpms. Relocated the fuel filter and fuel pump to accommodate 2 micron fuel filter and water separator bowl, and replaced fuel lines. Carburetors are marine 650 cfm edlebrock #1906. Secondaries look to be opening on throttle advance, throttle cables are responsive. I've spoken with two mechanics, both were on the boat and observed the issue and were as puzzled as I am. Only suggestion was to switch the carbs and see if the issue replicates on the port engine or fixes the issue on the starboard side.

At this point I'll take any and all possible troubleshooting suggestions, ty.

All points well taken.

Engines are both left handed. Atlantis propeller balanced the props last year, no issues there. I have a replacement set and will verify the ones in use (more aptly very little use).

Will definitely revisit the fuel filter and fuel flow/pressure (second mention of that being a possible source of the problem). Did not think that could be an issue with electric fuel pump working. Only the port engine had a new head, the other 3 were fluxed and redone. I did all the wrench turning and had help with adjusting valves and setting timing from 2 very experienced Boat mechanics.

Definitely not a street set up...in fact I had to undo many issues that a car mechanic who worked on the boat prior to my time. I have an older mechanic who is the proprietor of Mobile Marine and he has been very generous with his time and has forgotten more about marine systems than I'll ever know. Grateful for all the responses on the board, thank you.