A 429/460 uses a different bolt pattern then the 6.9L.
Personally id keep the 429....a lil work (which would be cheaper and eaiser then doing thr 6.9L), and you would have alot more power. Not that anything is wrong with the 6.9L, its a good engine, but kinda goin backwards from a 429. Plus if you want simplicity its harder to get simplier then what ya already got.....if you think swapping a diesel into a gas truck is simplier then running a carbed big block thats already there then you are confused my friend.
later,
Dustin