I found a running one here in a 71 t bird, and the whole car cost me $70. lol. they're out there.
Most Thunderbirds and LTDs had 429s or 460s in them
I don't think f250s had cobra jets? and the t birds asnd ltd will have thunderjets, in most cases..
and almost all your accessory brackets will need changed as they are typically attached to the sidewalls and fiirewall on a tbird or LTD, new motor mounts, and a c6 at least, if ur looking for an auto.
and
you just need big block motor mounts.. they should attatch just like small block ones... but theyre... bigger..
if its running, its not worth $700.... if its recently REBUILT, it might be. a stock 429, while a wicked powerplant, cant hold a candle to a well built small block... it weighs a ton more.
the swap will be expensive, for all accessories and bracketry... if I were you I'd reconsider.