Problem with your axles is they have 17 splines. The '64 has 28 splines and is considerably bigger and stronger. The 1965-'72 Spicer (Dana corp.) rear ends were stronger still and installed in place of the drop out (Salisbury style) rear ends because the 409 was breaking them far to often while under warranty for GM to make money on them.
You do not necessarily need to narrow the rear end depending upon your desire for a wheel and tire combination. The 12 bolt Chevy (made by Spicer but is in fact a modified Dana 44 rear end) is considerably stronger and lighter than a Ford made nine inch rear end. Just like there is a ten bolt as well as a twelve bolt the Ford nine inch has a weaker version that was used under all but high performance cars (351C and big block powered cars). So the odds of you finding a ultra rare Ford N case (nodular iron with extra reinforcing ribs and a formed steel housing instead of a weldment) are slim to none. Finally a Ford nine inch will cost you three to four percent of your flywheel horsepower to drive it due to the low centerline of the pinion.
Here are a couple of articles you might want to read.
How To Archives - Hot Rod Network
http://www.hemmings.com/mus/stories/2007/04/01/hmn_feature26.html
9-Inch Vs. 12-Bolt Rearend Compararison - Tech Article - Chevy High Performance Magazine
And if you are interested in installing a 12 bolt under your car here is a magazine article I scanned that outlines the procedure.
Sorry to say I lost the last page from Photobucket somehow. I have all of the common car mags published from 1960 on (and few uncommon ones such as Chevy Rumble and Cars) so if you need an article I can dig it up and scan it to send as an e-Mail if need be.
Big Dave