If you choose a small block, I recommend using the front end of the existing engine (timing chain cover, water pump) so you can mount the alternator and power steering (if equipped) and also use the original radiator. If you use the "newer style", your radiator outlet will be on the wrong side.
If you upgrade to a 340/360, you will have to get the left motor mount for the 340 as the ears on the block are different widths from 340/360 to the 273/318. The pass motor mount is the same for all small blocks.