There's a big difference between the 66 and 67. The 66 used the old 389 engine, and the automatic cars were 2-speed Powerglides. The 67 was the first year for the 400 with a Q-Jet carb (389 used the little AFB), and the 67 automatic cars were Turbo 400s - a very good combo. Your choice of which one to restore depends on your preference on doing a 389 car or a 400. The 67 is the last year for the early body style, so it's the most refined of the "boxy" classic GTOs. However, the 66 was the last year for the Tripower engine, so if you want a "Triple Power Pack" car, you need to look for a 66.
68 was the first year for the new body style, and 69 is very similar. Mechanically, the two cars are almost identical. 68 still has the side vent windows, and 69 is the first year for the eliminated vents. The 69 interior is a bit more refined with the high-back buckets and bigger fully padded dash. The '68, like the '67, had optional disc brakes, but they were the 2-piston design like a Vette. The 69 was the first year for the "modern" single piston caliper on the disc brake cars. Drum brakes were standard for both years. Both 68 and 69 are almost identical to the 67 in terms of the actual driveline components and systems. Engine optinos were slightly different between the years, with the Ram Air III & IV being available in '69. '68 had the Ram Air II, but these are very rare.
There are a lot of GTOs out there, but prices are no longer cheap.
Parts for all years are readily available from all the GTO restoration parts places. Just bring a truck full of money.
