I have used
http://5speeds.com/ as an information source for quite some time. I have as yet to purchase anything from them so I don't know the quality of their product, but information I've gotten from them has yet to be wrong.
I would immagine there is a factory gear set that would get you that ratio or close so don't rule out recycled gears.
You probably already know this, but the OD gears are in the extension housing and are held on with snap rings.
The extension housing comes off easier with the trans out of the car (as you have to take the shift rail block off the rail, don't loose the roll pin, or the bal bearing and spring that hide under the block).
As gearhead mentioned, the WC and NWC gears don't interchange, neither do the Ford spec V/S the Chevy spec OD gears (from what I've seen they're 1 spline different on the shaft
dont know if that's true for all years though).
Just out of curiosity, is it too big a split (for your setup) to shift out of 5th into 4th (1:1) when you are in the hills?
I know the WC I have in the Model A (.68 OD) isn't for the engine I have in there (off toppic cammed up bent 8) But I don't know how heavy your ride is either (guestimate, roughly 2800 empty for the A, probably close to 4K with Pops, I and all the crap we bring with).
Not throwing stones, just curious.