I did an AX-15 in my last '65 C10 and was very happy with it. I liked it enough that I bought one for my current C10 panel. It shifts nice but with the stock shift, they are a little long on throw but not too bad. There are aftermarket short throw shifters available.
As for speedo, I think early ones had a standard cable attachment and later ones were electronic. Luckily you can swap either style as I had swapped from electric to cable drive. Found the drive unit from an earlier Wrangler. You can use drive units out of a 4x4, which are much easier to find.
Only downside is you'll want to limit your HP and torque as they are good for about 300 of either. Oh and don't drive like an idiot, i.e. no clutch drops, power shifting and such.
If you want to see a thread about it, here's one I did
http://67-72chevytrucks.com/vboard/s...d.php?t=475370