Should I add Power Steering?

If I were you I wouldn't complicate the rebuild by converting to PS, you'll increase your costs 4X just with the ram, control valve, pump, brackets, hoses, etc. The I6 is lighter than the V8 so it'll be easier to turn relatively. I would do things to make the manual steering lighter like using a large steering wheel (like the stock one), not putting overly wide/short tires on the front (like 225/60/15) and staying with a stock ht./width (185 70 14?) just don't go larger than 195 I'd say even if you move to a 15" wheel.

You can also change to a roller idler arm (verify with John it'll fit the 6cyl.)
Opentracker Racing Products - Roller Idler Arms
which will free up some bind but you need a good alignment so the tires will return to center after a turn. Also install John's roller perches:
Opentracker Racing Products - Roller Spring Perches

Make sure the gearbox has fresh grease installed, again to eliminate bind.

Are you switching over to V8 steering parts? Which disc brakes are you installing? The I6 parts are a little different and thinner normally, always good to get out of the way should you upgrade to a V8 later.

I'd read this article before starting:
DazeCars, Suspension 101
and suggest doing the 1" Shelby/Arning drop at a min. and have it aligned to the shelby specs. It'll put the tires flatter to the ground for more contact patch, better steering and response, especially with the tires we have now compared to the original bias plys.

Again any of these roller items you can install to free up bind in the steering/suspension will make the car easier to steer and ride better/ respond better. Also teach him how to drive a manual steer car, just getting the tires rolling at <5mph makes a huge difference in turning the wheel for parking manuevers compared to trying to scrub the wheels from a stop.
Jon

+1 to what JonK said...lots of little tidbits that might help out (definately won't hurt things) and definately the Shelby/Arning drop...too easy to do while you have the suspension out (and it only lowers the front about half an inch).
I would however like to offer you another option, not sure if you have read about the borgenson power steering conversions. You can keep your manual linkage and this is an internal power steering box (much like chevy at the time) no complicated ram, control valve, hoses everywhere. And I think they start around 1000 bucks, less than it would cost you to buy all the parts for the factory conversion and get a much better feel for the road.

Borgeson Universal Company :: Ford Mustang 65-04 :: 1965-1970 Mustang Power Steering Conversion Kits :: Mustang P/S Kit, 1965-1966 289/302/351W Manual Steering

Just a thought, good luck with your project.

a six cylinder mustang is light enough on the front end to not need power steering. i have a 64 falcon with the six and it doesnt need power steering.

I have a V-8 and manual steering.

I did the Opentracker roller upgrades mentioned above and its not bad at all, if he has the rollers available for your car I highly recommend them as well.

My '65 was my first car when I was 16 as well, even with the original 289 with iron heads, and stock intake and exhaust manifolds, I had no problem driving with manual steering (I was all of about 140 pounds so I certainly wasn't packing a ton of muscle to steer). It got a bit tougher when i put on the 16" wheels and 225 tires 6 months after I got it....but today 15 years later.... I have bigger and way stickier tires and still have manual steering.

Your son will be fine.

I agree the extra parts for power steering can be a P.I.T.A, are more expensive and prone to leaking. I experienced this first hand the first time when my hoses went on me, then again when the ram started leaking.

When the ram went I got a replacement which was incorrect (too short) so I got fed up and tore out all the power steering bits and went arm-strong for the rest of that summer.

While I didn't have any problems driving around with armstrong steering, it sure was nice when I finally got around to fixing the power steering. I live in a city with tight streets and tight parking lots so it definately helps a lot. Cruising around is effortless and enjoyable instead of a workout