I could be wrong, but my understanding was that the Chev 409 was used in medium and heavy duty Chev trucks and the 534 was used in Ford MDT/HDT.

I'm assuming that the Ford 534 might of been the better truck engine, due to it's significantly greater displacement which would give more torque than the 409, I'm guessing.

I'm also thinking that as the Ford was a purpose built truck engine and the Chevy 409 did double duty as a car and truck engine, that Ford would of had a chance to make sure that the 534 design could be pure truck.

But not having had any experience with either engine, I leave it to those, that know.