His first winning contest was in 49. and he very sucessfully competed till the early '70's and still looked as good as he did in his 20's. Also he benched #500 at like #230-240. Now at that time, the other guys in that range were mostly atleast #90-100+ over his weight. He also had a great squat and dead, so he wasn't just a 1 lift lifter.
Of course he used. As to what? Everything that everyone else was using. Dianabol, Anadrol, Test, etc..... Just look at the popular drugs from each era and there you go.
I don't think Park ever claimed #1000 partial overheads, York claimed that for Grimek though. #800+ partial overhead, but when you see the pic, it's a high chain support, and the photo is angled so you can't see the plates well, but it's less than #800.
Look at Park through the years here-
http://musclememory.com/show.php?a=Park,+Reg .
Scroll to the bottom.