I don't have experience with either of the manuals from Rock Auto. After buying many, many Buick manuals for use on this site, I've learned some reproductions leave a lot to be desired on the quality front.
Pictures become fuzzy from scanning at too low a resolution. Images become blacked out with incorrect contrast settings. Pages come out crooked, missing, cut off, or in the wrong order due to careless scanning. Many manuals contain foldout complex color wiring diagrams etc that are not that easy to reproduce correctly in one piece.
Text documents frequently loose fine detail if stored as searchable digital files instead of as scanned images. Small print, fractions, graphics often drive the scanner crazy. [If you want to see an example, I can send a link to the Buick Master Parts catalogs (1940-72) that cover your car.]
Some manuals are extremely rare or costly, so using a reproduction is necessary. The original '64 manuals are still available at reasonable prices in good to excellent condition. I'll always take an original document over a reproduction. That's even if the original isn't pretty and has some grease stains and acid burns on it. The printing quality going from an original to a copy can only go down.
