Wait! I got the best solution....
Buy them BOTH!!
For $3000 you can put them both together to make one hell of a car.
Put the LT1 into the buick so you get muscle, vista window, leather, better paint choice, and woody incognito power.
Or put the interior into the yuk maroon (LOL) caprice and leave the engines alone. The LT1 is a much younger engine, and probably your best bet.
But why not have both? Can't you have your cake and eat it too?