I may be a bit too late in posting this but, '62BillT' is absolutely correct and having 'upgraded' the steering on my '64 with the Borgenson 'kit' from Ecklers, I can pass on some real world info.
There are 'issues' associated with this conversion.
1. The Borgenson 600 box mounts higher than the original box. This causes the pitman arm to be higher than original. This causes the drag link attachment to the pitman arm to be higher causing the driver's side tie rod to be angled downward towards the wheel AND to lengthened more than the right side to get proper alignment. This causes a left to right geometry difference that is not fully compensated by the required slight relocation of the idler arm. All this WILL result in some noticeable bump steer as you drive through moderate to sever dips. ....especially IF you do not have a sway bar.
2. and, although the steering will be more responsive, your turning radius will me increased significantly.
3. You can forget about getting the recommended 4-5 degrees of caster with the stock control arms. ...maybe more like 1.5 MAX.
It is FAR easier (not necessarily cheaper) to go with what was offered in '64.
I recommend either rebuilding your OEM box or getting a new/rebuilt one. (Manual and power assist boxes are the same).
Naturally you will need a pump, pump bracket, the four hydraulic lines, the power steering ram, the power steering drag link, control valve, and powers steering pitman arm, and crankshaft pulley, BUT EVERYTHING will bolt into place and when properly adjusted, and aligned to factory specs, will steer just fine, thank-you.
Keep in mind that this 'stock' steering is the same as used on my brother's '67 435/427 and my son's '67 RS Camaro and it works fine.