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.
F250s Got mine out of a 94 didnt have the "roller setup" in it at the time but was the correct "Roller" Block with the two threaded holes in the intake valley. The EFI intake can be quite tricky to get off but is best way to tell without pulling the motor. just make sure you have a assortment of torex tools