Anyone know if the turning radius can be adjusted by the internal stops? I've never had one apart so know nothing about them.

From what I've been able cipher, after hours of searching on the computer, the end cap is changed to modify turning radius one way and internal snap rings are used to modify the other directional stop.
Not sure how and can't find an explanation, hope someone on this sight has knowledge they can pass on.

Al, I know your part supply is looking mighty Ted where you are but maybe mail order can help. I bought a box out of a 98 Grand Cherokee. It's got nearly the same exact same radius as the std box. It's got more steering effort so you get better road feel. It's not quite the same as one of the IROC boxes but it was easy to get and I am very happy with the swap. It is a metric box so you have to get some adapter fittings but they are easy to get.

Notes from what I used. Steering gear is faster than stock, at least what my '68 came with, but isn't so fast that it's twitchy. (I'm still using a big, factory steering wheel) I understand the General used several combinations of steering gear, pitman arm length and different length steering arms. A bunch of info can be found on Pozzi's web site.

I'll forward my entire front suspension write up including photos if you PM me with where to send. Be patient, I'm having computer issues and it may take a few days to respond. My car is a driver, not an race car of any sort.

Front Suspension & Steering

Steering system rebuild in spring of 2010 resulted in huge improvement in system operation. Original ~ 3 ¾ turns, 17.5:1 ratio steering gear, dated 079-8 or the 79th day of 1968, replaced with a NAPA reconditioned 2 ¾ turns, 15.23:1 ratio steering gear. Faster ratio is an improvement over stock gear box but isn’t so fast that steering is “twitchy”.
• Steering Gear: NAPA #88277060 Cap bolts replaced with 3/8-16 x 1” long button head cap screws for more header tube clearance.
• 5 ¾” (fast ratio) pitman arm used Moog Part No. K6582. (reinstalled pitman arm – good condition)
• Magnefine in-line filter Napa Part No. NSP 81FLT2
• Imperial Oil Cooler Part No. 241009. Purchased from Advance Auto Parts
• Pressure hose NAPA Part No. NPS 76000 Note: 4.1L 250 CID Pressure Hose matched original.
• Fabricated 3/8” steel return line from steering gear to filter and oil cooler.
• Lares steering coupler (rag joint) Part No. 201 purchased from Advance Auto Parts.
• Rebuilt power steering pump, Part No. 20-6117, purchased from Advanced Auto Parts. Reused original pump housing and cap.
• GM power steering fluid Part No. 89021182 (quart) Supersedes GM Part No. 1050017 and more recently 89020661 Note: Parts Department at Schepel GMC very helpful. Open Saturdays 7-12 (219) 769-7757
• Reused relatively new stock idler arm Moog Part No. K6099
• Stock 15/16” diameter center link with Moog ES-381RL Outer Tie Rod Ends; Moog ES-681N Inner Tie Rod Ends and UMI Performance steel tie rod sleeves Part No. UMI-2102
Installation Notes:
Bleeding power steering system as recommended by retired GM engineer worked well. With wheels off the ground, turn steering wheel lock to lock slowly until system appears bled then start motor to finish.
Frontend alignment performed by Hirata’s Lowell Automotive & Radiator on June 3rd 2014.
Stock GM Alignment Specifications:
• Caster +1/2 deg
• Camber + 1/4 to 1/2 deg
• Toe-in 1/8" to 1/4" (Radial tires need less toe-in, use 1/8" or less for them)

David's Pozzi Racing Recommendation
• Caster 5 deg positive, or as much positive as you can achieve up to 5 deg, can use .5 deg additional positive on the passenger side to compensate for road crown. A common setting would be: LF 5 deg positive, RF 5.5 deg positive.
• Camber -.25 degee for mostly freeway driving,
• -.5 for street and frequent hard cornering or mountain roads,
• -1 deg for street, Autocross, and Open Track, this will wear the inner tire tread if you drive a lot of freeways. I use -1.5 deg quite often on cars that we frequently autox and do only a little street driving. you can use even more if tire temps show outside edge of tread is hotter than inside after a track run. If the inner edges of tread show excessive wear, use less neg camber. An autox only car would use -2 to -3 degrees. If you have not done the Guldstrand mod, you can be more agressive with negative camber and increase neg camber by another -.25 to -.5 degrees.
• Toe in, 1/16" to 1/8".

Good information
I've done these alignments
For 99% of firebird owners I would stick to + 2 to 5 degrees caster and .25 camber with the split for road crown and 1/16" toe if using radial tires.

It took a marathon in front of the computer but I did find some info on the internal stops. The amount of travel is limited by using different thicknesses of end caps, to stop the piston travel in one direction, and a spacer to stop travel in the other. The caps and spacers come as a matched set with the thinner allowing more travel and the thicker allowing less. The thinnest cap has no matching spacer.
Now I have to find someone to get these parts from and also determine if custom travel can be had.

I also found the amount of road feel is determined by the thickness of the torsion bar. The thicker the bar the more road feel before power assist is started. Something to thick about if one is happy with the turning ratio but wants a firmer feel. I'm happy with my 3-1/2 turns but I would like a bit of a firmer wheel.

Interesting. It seems to me that I've heard guys say they have adjusted the steering effort( road feel ) by a pressure limiting valve in the pump.

Yes I've heard that as well, but flow is what's important. A fast ratio steering gear should have a flow of 2.75 GPM. If one changes the gear from a slow ratio to a fast ratio and keeps his/her old pump, the orifice in the flow control valve can be drilled out to 0.148" to get the proper flow. The pressure relief valve should cut out at 1,250PSI. The pressure could be increased by puting shims under the spring or installing a new spring, but when the steering is against the stops how much pressure will the hose take before it blows fluid all over the engine, and hot headers?

Had a flash engine compartment fire in my old F-150 for exactly that reason. The high pressure hose blew.