One can build a SERIOUS street machine without 'cutting up" the car. Most of the performance parts available are intended for installation in unmodified cars. If all the stuff is "bolt-in", you won't harm the integrity of the car.
Be sure you're aware, there is no "need" for anything but a Pontiac V8 today. ALL the "goodies" for a hotrod engine are readily available, and easily puts the Pontiac "out of reach" of a small block, and it takes a WHOLE LOT of "big block" to make the kind of streetable power the Pontiac can make. Many will "bend your ear" to put a Chevy in it. They're "scared" of the Pontiac when "done right". 600 HP on 93 is a "slam dunk" today. And they are as or MORE reliable than an equal BBC.
A LeMans or Tempest doesn't command the resale value GTO does.