Nothing to gain with the later 340 heads - 587 J head 1.88 intake probably same or similar to whats on the 360 already(unless they are already built, of course..). Provided you get everything truck specific for the 400 - pan, motor mounts, radiator, etc. I'd do the 400. Otherwise stick with the 360. A lot depends on what the 400 is coming out and can donate to the swap. I've done a few 727's and could probably tackle the tail swap with a good service manual. Best to ask questions on that swap for whatever small differences might exist year to year in the transmissions. If you're paying someone make sure to ask those kind of questions to see if they are on their game.