While you have gotten some good suggestions here I really doubt you will find a 429 from a boss or even a lightning or saleen in a boneyard.
Stick with 94-97 f150-250 an econoline vans. You have a fairly decent chance of getting a roller block. If you do a search on google for "351 roller block" you will find a link to casting codes for the rollers blocks. This way you just pull the starter (instead of the intake and the mess associated with efi) to verify.