My first 2 cars, a 64 Special with the V6 and a 65 Skylark, a 2-door post with the 300-in 2bbl, and yes the quarterpanels and trunk floor were particularly rust prone on those Buicks, especially after some NY winters, learned that the hard way. I’ve seen a Special with a 455 in it, it’s doable, as long as the brakes and suspension are beefed up to handle the extra torque. When I had my 65 there was a local junkyard that had a 65 Gran Sport, I started pulling out the buckets seats and console and make my Skylark in to a GS clone. But the damn timing chain jumped a gear and the engine caught fire. That was the one problem with the small Buick V8, the plastic timing gears wore out fast otherwise they were good engines. No damage, but I was so frustrated with it I told my brother-in-law to give my $25 for the bleeping POS and get it out of my sight. To this day whenever I see a mid-60’s Skylark at a show or cruise night I kick myself. But I’m happy with the Impala :)
I was flipping through the newspaper one day and saw an ad for a '64 Impala (non SS), supposedly with a 409.
Called my buddy up about it.
A few hours later he pulled up with his 409.
Took the best of both cars and built a '64 SS 409.
Then life happened. It was in the back of a garage for a number of years waiting to get completely gone through. 20 years and 7 kids later, he traded it away, never able to restore it the way he wanted.
Wasn't a four speed (more like powerglide), dual quad, positraction, 409.