Sounds like a cool project, I can hear the purist crying already!
We recently completed a 67 in the same manor for a customer that wanted much more out of his GTO.
There are a lot of major benefits from going with a complete chassis vs bolting on components to a stock frame. For starters, you have new metal vs a 40 year old frame. All of our chassis are designed to give great strength gains over the stock frame which is definitely needed. A stock frame has set mount points for suspension which limits the geometry and only allows you to go so far. With our Chevelle/GTO chassis we did the exact opposite. We started with the suspension geometry that we wanted and then built the frame around that. Many aspects of the build are taken into account in our chassis to ease the build process. Stock body mounts, bumper mounts, core support mounts, are all in the correct, factory location. Engine placement has been set so no modification is needed to the firewall. The chassis has been designed with clearance for full length headers and 3" exhaust all the way back. Rear frame rails are narrowed to accomodate a 345 series rear tire.
There would also be a considerable amount of fabrication required to get the strength and torsional rigidity out of an original frame. With a stock frame, you are also limited on ride heights, steering, and tire sizes... taller ride height and narrower tires with a steering box instead of a rack and pinion.
It all depends on what your plans are for the car and what your budget is. Definitely mention that this will be for a GTO because the GTO frame is considerably different than a Chevelle frame.
As far as performance goes, we have built several cars as test beds for our chassis to test the real world and track performance. This has also helped make several key component changes (spring rates, sway bar thicknesses, etc...) after driving to dial in the best mix of ride quality and performance handling. Our 70 Chevelle has proven that a big body Chevelle can run with the stripped down Camaros and continually place at the top in the pro-touring events.
Give me a call or shoot me an email if you have any questions.
-Phil