273 or 225?

Yes, and your car should already have a slip yoke 904 in it since it's a 66. If you use a newer than 65 904 and bolt any small block V8 in the car attached to the 904, your existing driveshaft should work right back. Later, if you upgrade the rearend to an 8 1/4 or 8 3/4, you will still be able to use the same driveshaft...but will have to get it shortened a little. Ball and Trunnion driveshafts stopped in 65. Any 66 A body had a slip yoke transmission, be it auto or straight drive.
Could that be avoided by getting a later v8 904?
It depends on the year of the V-8 and the year of the trans. The 904 converter pilot got larger in 1968. So, a 68 or newer 904 will not fit the crank of a 1967 or older V-8 (same for the slant six)