the Buick 455 shares most of the same design principles as the Buick 350 and would be the easiest non-SBB swap. you'd need new frame pad mounts and a new fan shroud and maybe a couple of other things.
www.TAPerformance.com can answer all of your questions and has their catalog online.
depending on which alternate motor you tried to swap in there can be quite a bit of variation with the Buick layout with respect to things like electrical, starter, distributor, etc.
if you put in a Chevy, you'll need to grab a trans with the motor. GM had two different bell housing patterns, the Chevy style and the BOPC ( Buick, Olds, Pontiac, Cadillac ) style.
1970 Caddy 500? 
the largest OEM engine available for the Le Sabre in 78 was an Olds 403. not sure you want to put a 403 in that ( weak block ), but if you got the 403 mounting hardware, dropping in an Olds 455 / 400 would probably be the next best option.
https://en.wikipedia.org/wiki/Buick_...2.80.931985.29
the mid-80s Buick turbo 6 is the same basic block as the Buick 350 - 2 cylinders, so that's an option too.
The way to crush the bourgeoisie is to grind them between the millstones of taxation and inflation.
Vladimir Lenin
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H.H. Goddard, Human Efficiency (1920)
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