I installed front and back ’06 GTO seats in a 69 along with the seat belts. Very time consuming to say the least but they look great I think. As you mentioned, the front seats are way too high if you don’t modify them. I fought for every 1/8” I could. Because of this adjusting them up and down was basically out of the question as they were barely low enough when all the way down.
Most of the mods were on the seat tracks/mounts. You can see where I cut off the feet and rewelded them. I also had to make dents in the floor seat structure to get the adjustment motors to clear. I fabbed new seat mounts to the floor seat structures to allow the GTO seat pattern to bolt in properly. A part of the outer seat structure had to be cut away from the rocker and rewelded for clearance of the seats. I also had to modify the side seat trim where the seat controls are by trimming the lower edge to clear the rockers. That also meant relocating the control box. I also didn’t use the sheet metal covers that go on top of the rockers. And, I bent over the rocker pinch weld seams to get more clearance. I also had to modify the harness so all the wires would lay flat like a ribbon cable on top of the driver side rocker.
I even went as far as cutting out some of the seat foam at the rear of the cushion to make it thinner. The console was also interfering with the foam frame that supports the seat bolsters. I cut those out completely as the console did a good job of supporting the bolster foam. I was using a custom console that was based on the shape of the factory console. A factory console might not have this issue. I even found someone to stitch in a new back panel to replace the GTO emblem.
And, finally, I borrowed from the 5th Gen Camaro the shoulder harness guide design. I had to partially disassemble the seat back to gain access to the this area to secure the straps. I used snaps instead of magnets. Keep in mind when you pull off the back panel the plastic fasteners will probably break every time. News ones are, (were), available.
You didn’t mention the back seats but the GTO’s are too wide and tall. I had to cut the seat back wire frame in the middle to and at the bottom. I cut the foam as required and tucked the excess material out of the way. I had to weld in new wire and modify the the factory floor brackets to secure the bottom cushions. I also eliminated the seat back brackets/hooks and fabricated new ones to match the GTO wire frame. I used a single bolt between the backs to help secure it a little better. The bottom bolts were extremely hard to get to but I got them to screw in one flat at a time. And, with the front seats adjusted for a typical sized person, there is no room for someone's legs in the back seats. Kids only!
The seat belts were slightly modified too. I disassembled the reels to re-clock the pendulum mechanisms as the reels were now installed at different angles in the Camaro. I made some sheet metal armrests similar to a deluxe interior/convertible style to house the front seat reels. The back reels were hidden in the vertical portion of the armrests. I used the factory front seat shoulder belt mounts. For the back I drilled a new hole and reinforced it near the back glass for the back seat shoulder harness mount. The rear belts were relocated to the tunnel and floor with factory reinforcements.
The front outboard seat belt mounts were bent to allow them to bolt in the factory holes. The only thing I didn’t like about this was that it would probably straighten out in a crash and lengthen the belts but I figured it wouldn’t be enough to cause problems.
This particular GTO was in a head-on crash so the front seat belt tensioners were activated. I had to take them apart to get a reasonable amount of slack to allow the buckles to be accessible between the seats and console. I mounted those in the factory locations.
So, you can see it was extremely involved and a PITA in retrospect. I only did it because I figured I could and wanted to prove I could! Besides the engine compartment, the seats and harnesses are what I am most proud of. I never did anything like this before. In fact the entire project was way outside my comfort zone but it all came together quite well. Sorry I don't have more photos of the finished product.
Anyway, I’m not sure if I would do it again! Hope this helps whether I am encouraging or discouraging you to do this.