318 LA rebuild after losing a valve seat

Those are the "late"rods, the heavier duty rods. They share the same basic dimensions with 340s and 360s.
They will be the same dimensions as what would have come from the factory on your 73 crank.
My "645" rods have been bouncing off the rev-limiter at 7000, since 1999, in my 360. If you keep the engine out of detonation, and the engine doesn't ever gulp water, then they should be the least of your worries.