cheap!..
I'd like to have a 455 Buick motor for 200 bucks,given the choices you listed as far as the prices..they go in chevy trucks easier than a caddy ,from what I've seen and heard, the motor mounts are the only dilema,the oil pan clears the frame without mods or changing it..they are tourque monsters, even stock they put out over 500 ft lbs !..
I'd say buy it,it WILL bolt to any TH400 Caddy,Pontiac,or Oldsmobile tranny after 1967..I'd leave a 4 bbl carb on it,for economy,and just plain pleasure of hearing it roar!...it would make an awesome hot rod motor..you CAN get some stuff like intakes,cams,and headers for buicks,they just cost more and you have to look harder than you would for Chevy parts..not sure about injection,but they did have Hillborn's on the Nailhead motors!..
Older Buick "Nailheads" also had a TH400 with a "switch pitch" torque converter,that had a round bellhousing,that looks like a fire hydrant where it bolts to the pipe,they wont fit later motors with the "V" style bellhousing..
I had 2 of them,401 V-8's with the TH400..they fried the tires quite easily in the "high stall" mode.

--I rigged the detent and stall speed switches into the drivers armrest,so I could use passing gear,or change the pitch on the torque converter blades with the push of a button!..almost made the TH400 feel like a six speed,instead of three..I wish later tranny's kept that style torque converter..it CAN be swapped in I guesss,but good luck finding one for a donor,only made them from 65-67 in Buicks and very few Olds and Pontiacs..
The "Nailheads" are VERY powerful and torquey,and have a fairly short stroke,so the can wind out a bit higher than most other big blocks without tossing a rod..mine would bark the tires at 65 mph shifting into second gear wide open in a 6000 lb 66 Electra 225!~..I'd hate to see what it would do in a light car!..they would make a great off road stump puller too,but they are getting rare now...
The 455 Buick is a more "modern"long stroke /low rpm torque motor,and a bit more plentiful...I'd say they were good engines,I never got calls for engine parts for buicks other than gaskets mostly..so either they last forever,or people junk them and don't rebuild them if they do crap out I guess..
