The 350 and 327 are the same 4.0" bore. The only difference in the large journal 327 and the 350 is the stroke of the crank. The rods on both engines are 5.7". The crank from the 350 will go directly into the large journal 327 block and the 327 crank will go directly into the 350 block without any clearance issues. Pistons will be different between the 327 and the 350 however due to the stroke difference in the engines requiring different wrist pin heights - so the pistons need to stay with the crank.
The rods on the 327 and 350 are both 5.7" center to center. Since the stroke on the 327 crank is shorter the piston wrist pins are in a different location then they are on the 350 pistons to make up for the shorter stroke. If you used the 350 pistons with the 327 crank and rods then the pistons would sit too far below the deck and you would have a low compression ratio. If you used the 350 crank with the 327 pistons the pistons would hit the cylinder heads.