Rand.....you can put a Welch plug in the back of the early block and run a PCV system with no problem. There are kits to convert the canister filter to a spin-on filter as well. The early 327s were virtually indestructible on the bottom end because of the stronger crank. If you want the later block, any 350 will work. The only difference in a 350 and a 327 is the crank and the pistons, but you must use a large-journal crank with a 350 block. All had 5.7" rods.
As far as engine mounts, they were all the same physically except for the Z/28 and 350 as far as I know. The higher-power engines used an interlock in the rubber instead of just solid rubber. As for me, I use the urethane mounts. They are stronger than the rubber mounts and they won't rot. They don't look original, but as for me they are far superior. The frame mounts are all the same as far as how everything bolts up.
One other thing--the early 327s used a different, smaller harmonic balancer and the timing tabs on the timing cover were different because of the diameter and welded to the cover. Later engines used a bolt-on tab. With a hotrod buildup this becomes a moot point with speed parts, but it's one of the anomalies.
Again....good luck and I hope this helps!