For anyone who is interested, have resolved my issues with my bowed play field and I thought I should post the resolution, and what I have learned.
1st, what did NOT work was just installing the rails. All installing the rails did was straighten out the sides of the play field while leaving the center area of the play field bowed. Even after I left the play field clamped flat to some support bars for a few weeks, it would still bow up as soon as I removed the clamps.
2nd, with the play field supported above my work table on some blocks, I observed that I could press on both the top and bottom of the play field and my weight would force the play field to straighten out.
3rd, part of the solution is already built into the cabinet by the manufacturers, so when the professional play field restorers say they don't worry about warped play fields, it goes to the way cab is built. As I explain what I did, I hope it also explains how the cabinet is designed to compensate for bowed play fields.
I'll try to explain..............
With these old Sterns that I have, the play field is supported inside the cab by two "Z" brackets on each side of the cab. There are also two "H" brackets that lock the play field to the lock down bar receiver. At the top end of the cab there is a strip of wood that goes all the way across and on the lower side of this wood strip are two rubber bumpers that apply force to the play field rail at the top of the play field. These two rubber bumpers, the two "H" brackets at the lockdown bar and the "Z" brackets all apply pressure to the play field and hold it in place and keep it from rattling around when you are shipping a pin all the place.
In theory, I think these two bumpers, the "H" brackets, the "Z" brackets---and the side rails are all supposed to apply pressure to keep a play field from going into a bowed position inside the cab.
I installed the two "H" brackets to my naked (side rails not installed) play field then set play field in the cab on the "Z" brackets. To the naked eye, it all looked fine but when I laid my straight edge to the play field, the bow in the field stood out like a sore thumb.
My first move in correcting the bow was to screw a 15" long piece of wood to each side of the cab to add more support to the play field than what was being offered by the "Z" brackets. This is the way the old Bally's are set up for play field side support inside the cab.
With these two new pieces of wood installed for side support, I laid the play field back into the cab. With these new supports, the play field rocked back and forth like a rocking chair. Plus, the "H" brackets were up in the air and not resting on the lock down bar receiver as they should have been.
These "H" brackets will be forced down to the receiver when the lockdown bar is positioned, but this lets you see the bowing problem.
The second move I made was to add a length of aluminum "L" angle to the top end of the play field. This "L" angle is measured to contact that strip of wood that goes across the top of the cab and force the top of the play field into flat position all the way across. With my "L" angle installed, the two factory rubber bumpers were no longer needed and they were removed.
(NOTE: This "L" angle offers another side benefit, too: It protects the two arch plastics from dragging and possibly breaking when you move to lift your play field).
This shows the "L" angle sort of lurking in the back behind the top rail.
With the top of the play field being held down with the "L" angle, I can apply pressure to the bottom end and force the "H" brackets to the lock down receiver. This forces the play field flat and it loses the bow. Now, when I install the lockdown bar, the play field is forced into flat position and my bowed play field problem disappears.
I suppose it could be argued that I was tilting at windmills and making mountains of molehills and that the problem would have corrected itself. But what I have here is a sure thing and now I can rest easy as I start putting this pin back together.
I want to thank everybody who offered ideas and thoughts on this issue.