Shouldn't be a big deal, but a few things you'll have to watch out for.
First of all, you'll have to get the motor mounts & brackets. I believe that the Caprice is on the same platform as the LeSabre / Wildcat etc., so the swap should be pretty easy.
You'll also have to mess with the wiring a bit- for example, the distributor is typically located in the rear of the engine on a Chevy, it'll be on the front on the Buick engine. Also, your car may be set up for HEI, and the 455 probably has points (it would be a good idea to switch to HEI anyhow).
The exhaust will have to be messed with, too- the exhaust manifold are in a different location on the Buick than used to be in your Chevy. No big deal for most exhaust shops.
Besides that, it should be fairly straightforward- maybe a few other small things along the way like radiator connections, tranny mount, etc., but I think you will be VERY happy with the new engine.
-Bob Cunningham
bobc@gnttype.org