The P-38 to, heavy load reportedly up to 5600lbs that could include 10 rockets, very stable and later models had dive slats that helped control speed. Those that used it that way said it was a great divebomber.
Another was the F4U-4 Coursair.
The Skyraider came later and was better at but was designed for it to.
Both the A-36 and the P-51 had a 2,000lb load limit, 1,000lbs per wing. It wasn't designed to be heavy duty or a heavy load carrier it was designed to be fast and light, and it was.
I don't know but will ck, later Spits had provisions added, I think, for extra fuel or small bombs. I doubt very much that they would exceed the P-51s capabilities.
The "dive brakes" on the P-38 were in reality two slats hinged in the middle and placed on the bottom of the wing at the 1/3 chord point just outside the engine nacelles/booms. Each "slat" was about 4" wide and 3' long, the lay ed flat, flush with the wing (retrofit slats were on the wing creating a small bump), when deployed they made a V below the wing. In a dive from high altitude they did two things: first they created drag, second, they interupted the airflow under the wing destroying the lift (negative) that was created there, counteracting the compressability on the wing. In normal flight it would "pop" the nose up 10 to 15 degrees which can be useful if your in a maneuver fight and need just a little to get on target. Finlay the P-38 was a very clean aircraft and anything that created drag was useful esp in dives, but the dive slats did nothing to increase lift.
My understanding of that was that a. they didn't always deploy evenly or correctly and b. they had a tendency to remain in the airstream unevenly at times. The maintenance required to keep them in proper working order was not always done so they were wired closed.