I haven't read Jim Hands book, but probably a good place to start if you are new to all of this.
The problem with the 350 is not that it won't make good power, I proved that to myself at the racetrack. The challenge is getting it to make good power, and bottom end torque, and be streetable down the interstate- all at the same time. Also there is much less good info available on how to set up a 350 compared to the 400 and 455 engines.
So what heads are suitable for a 350... all 350-455 CI heads will physically fit, but that doesn't mean that they will work well.
The 6X-8 heads are the 101 cc ones, right? If the pistons are stock flat tops then the CR with 101 cc heads would be extremely low (without milling them) which would be very bad for performance. A 72 cc head is about right for a 350 with flat top pistons running today's pump gas. I also learned that you have to be careful with big valves on a 350 engine since the distance across the valves can actually be bigger than the bore, which limits how much valve lift you can have without cutting reliefs. Too much lift and the valves will hit the cylinder walls. And most of the newer-style cams have much higher lifts than the original Pontiac grinds.
If you haven't already found one here is a link to a site that lists stats for all the Pont heads:
http://www.angelfire.com/on/geebjen/heads.html
I built my current 350 about 15 years ago and it is pretty well worn out. It has '67 #061 heads, 72 cc with 2.11/1.77 valves. I cut reliefs in the bores for the big valves and installed a Crower cam with 210/221 duration. The rest of the build was pretty much standard performance stuff. This engine was an attempt at finding "the middle ground" since I wanted to run a street gear. Results were mixed. Off the line with the 'vette converter its probably not much stronger than a stock 60's vintage 2bbl, but it pulls hard once it gets moving and the redline is up around 6000. With a big gear and converter it might even turn in some respectable times. But then it wouldn't be much fun on the highway.
More cubes makes things a lot easier and in the end, I think building the 455 would make you happier.
Walt